Thank you both for your encouragement about this study! Here's what else I've learned. Strangely, when I looked up all the lists of all the tribes of Israel in the Bible, Besides Dan missing from the list in Revelation, I found one list in 1 Chronicles Chapter 2 through chapter 8 that was missing both the tribe of Zebulun and the tribe of Dan. No reason is given but one commentary suggested that there may not have been any information on those tribes because of the chaos of the Assyrian invasion at that time. While that sounds reasonable, the only thing we can know for sure is that God left them out for reasons of His own and we may not know why till we get to heaven. Then in another list in 1 Chronicles 27:16–22 Gad and Asher are left out for no apparent reason! The other interesting thing about the lists is that the tribes aren't always (hardly ever) listed in the same order either.
So what we learn from looking up all the tribes is that God leaves some of them out sometimes without telling us why.
Next I read one of the major stories about the tribe of Dan. Remember that God told the Jews that He had chosen a specific area for each one of the tribes. The tribes were to go to the area He had chosen for them and kill off all the people there and take over the land for themselves. This was God's judgment against the gentiles for their sins which they had refused to repent of for 400 years. Well, Dan didn't have any luck getting rid of the folks that lived in the area that had been assigned to them. So without praying or asking an elder or priest for advice, they took off and found a place far to the north that they decided they liked. Problem was that God had not told anyone to kill or harm the people who lived there. They were a quiet people who minded their own business. But before they went to that city, they stopped by a house in Ephraim's territory and discovered this guy had bought a priest (a bad sin on both his part and the priests part!) He had hired him to be his personal priest and had set up a bunch of idols as his God. So the guys from Dan asked the priest if their mission would be successful and of course the priest said "yes". (not because he had really heard from God, but because he wanted to keep his head.) So the guys from Dan left and eventually found the town of Laish and decided that's where they wanted to live. So they got the rest of their tribe and again stopped at the house of the "priest", grabbed him and all the idols and took him along with them to the Laish and attacked it and won. They rebuilt it after the war and renamed it Dan.
The first obvious sin we see is that they didn't do what God had told them to do, and didn't even try to do it in His strength, but instead tried in their own strength and failed.
Next is Covetousness. Instead of taking the land God had for them, they coveted somebody else’s land instead and took it in a violent manner.
Then they took their "spiritual counsel" from an idolatrous and treasonous priest. Instead of godly counsel they got ungodly counsel.
Next, in today's world they could have been charged with Breaking and entering, robbery and intimidation when they stole the idols and made the "priest' go with them, and all of those are sins too of course.
Then there were the sins violence and murder.
Finally it all ends with idolatry. Let me share a bit from a commentary: The tribe of Dan was the first tribe in Israel to officially adopt an idolatrous system of religion. Even though there was a house of God in Shiloh, they preferred their images and idols. Years later, when the kingdom divided, Jeroboam I of Israel would set up golden calves in Dan and Beersheba and encourage the whole nation to turn away from the true and living God (1 Kings 12:25–33). You can find this story in Judges 18 if you'd like to read it for yourselves.
Many of the commentaries suggest that Dan is left out in the list of Revelation and the one I mentioned at the start of this post due to their idolatry. What bothers me about that is that eventually, ALL the tribes became idolatrous, regardless of who was "first". Maybe the people in the tribe of Dan were worse than the others, but then, maybe they weren't. God doesn't say and therefore we can't pass judgment on them that way. Since this did start out trying to discover why Dan was left out of the list in Revelation, let me share what some of the commentaries say about that:
The tribe of Dan is not included in the tribe list in Rev 7:4–8. This list has a unique order and does not correspond to any of the lists in the Old Testament. The tribes appear to be rearranged to place Judah at the top, likely because the Messiah was to come from the tribe of Judah. However, it is troubling that Joseph is included and Dan is omitted. Several theories have been proposed concerning the tribe’s omission:
• Bauckham suggests that the “list is an attempt to list the tribes in an intelligible order which failed owing to faulty memory”. I don't buy this simply because this list, like everything, or should I say, "every word" in the Bible was inspired by the Holy Spirit and He doesn't make mistakes!
• A Jewish tradition relates that Dan was the most apostate tribe.
• Some explain the omission of the tribe of Dan as a hint that the antichrist will come from the tribe.
• Since Dan is also missing from the genealogies in 1 Chronicles, the tribe of Dan may have failed to return from captivity and therefore lost its place among the tribes. This seems the most reasonable of these theories.
Lexham dictionary
Apparently, Dan was among the tribes that were the least supportive of the Israelite tribes. The Song of Deborah, which celebrates the Israelite victory over the Canaanite king Jabin and his mighty general Sisera, reproves the tribes of Gilead, Dan, and Asher. Dan’s apparent lack of interest in assisting the other tribes suggests that Dan, situated on Israel’s northernmost border, had more in common with its foreign neighbors to the north than with Israel’s other tribes.
The exclusion of the tribe of Dan from the sealing of the twelve tribes (Rev. 7:5–8) should not be overlooked. It appears that Dan had been cut off from the other tribes of Israel. However, Ezekiel prophesied a “portion for Dan” (Ezek. 48:1). Nelson’s new illustrated Bible dictionary. This is all good information to know and seems to agree with the last one of the theories above.
Much speculation has arisen about why the tribe of Dan is omitted. Joseph and one of his two sons, Manasseh, are listed, but Ephraim, Joseph’s other son, is omitted. Thus if Dan were included, there would have been 13 tribes. According to J.B. Smith, Scripture contains 29 lists of the tribes of Israel in the Old and New Testaments and in no case are more than 12 tribes mentioned (A Revelation of Jesus Christ, p. 130). The tribe omitted was usually Levi, from which the priesthood came. Inasmuch as it is normal to have only 12 and not 13 tribes, the omission of Dan is not significant. Perhaps Dan was omitted here because it was one of the first tribes to go into idolatry (Jud. 18:30; cf. 1 Kings 12:28–29). However, Dan is mentioned in Ezekiel 48:2 in the millennial land distribution. The Bible Knowledge Commentary
The fact that Levi is included in the enumeration has intrigued readers of the Apocalypse (he was never given a portion in the land under Joshua, only forty-eight cities in the territories of the other tribes). Also, the list substitutes Joseph for Ephraim; and, most puzzling, it omits Dan. There is no ground for dogmatic assertion here, although many opinions have been offered to explain the omission.
One position is that Dan is omitted because the Antichrist will come from that tribe, judging from Genesis 49:17. Built on so many imponderables, such a view can scarcely be called valid. Notice also, 49:16 is a strong promise. Another explanation is that Dan does not appear in the list because it was the first tribe to embrace idolatry (cf. Jud 18). But nowhere is this serious departure from the Lord evaluated as worse than the idolatries of the other tribes. Moreover, in the distribution of the land in the reign of Christ in Jerusalem, Dan is in fact given his inheritance (Ezk 48:1–2). KJV Bible Commentary
This next one just makes me shake my head in wonder of what some people come up with!
The list of tribes does not agree with any of the twenty-plus lists found in the OT. It especially does not agree with Ezek. 48:2–7, which lists the eschatological people of God. Dan is omitted, Judah is listed first, Ephraim is left out, but Joseph is listed in its place and Levi is included with the other tribes. Any Jew would recognize that this list is irregular and meant to be taken symbolically. Maybe it's because the guy is post trib that he can't understand a lot of Revelation. 
Hope in Hard Times - The Final Curtain: Revelation
These tribes may have been omitted due to their propensity toward idolatry (e.g., Judg 17–18; Hos 4:17–19). John may be trying to combat the churches’ participation in idolatrous practices (see Rev 2:20) In that instance, Dan and Ephraim make excellent foils. Faithlife Study Bible
(Joseph, however, being substituted for Dan; whether because Antichrist is to come from Dan, or because Dan is to be Antichrist’s especial tool [ARETAS, tenth century], compare Ge 49:17; Je 8:16; Am 8:14; just as there was a Judas among the Twelve) Commentary Critical and Explanatory on the Whole Bible
In this list the tribe of Dan is omitted, perhaps because they were greatly addicted to idolatry; and the order of tribes is altered, perhaps as they had been more or less faithful to God.
OR:
The omission of the tribe of Dan from the list is considered by some an indicator that the Antichrist will be of that tribe (cf. Gen. 49:17; Jer. 8:16). Alternatively, it may be that both Dan and Ephraim are omitted because of their involvement in idolatry.
OR (and I hesitate to even include this one it's so ludicrous!)
That ethnic Israel is not in view is confirmed by the irregular listing of the tribes. Judah is placed first as the Messiah’s own tribe, Dan is omitted, Levi is included as an ordinary tribe and Joseph replaces Ephraim.
Revelation, four views: a parallel commentary.
The peculiarities of the listing are these: (1) inclusion of Levi (v. 7), doubtless because of the cessation of O.T. priesthood; and (2) the substitution of Manasseh for Dan (v. 6), the latter omitted possibly because of its idolatrous history. Believer’s Study Bible Interesting reason for the inclusion of Levi...that makes sense at least!
And this last one I hadn't even seen till now, and it's interesting too, agreeing with some of the others we've already read:
Joseph’s two sons, Ephraim and Manasseh, step in and take the place of Joseph. But if Joseph is replaced by two in this list and yet the number of tribes remains twelve, a tribe must be missing. The missing tribe is Dan.
Why? The Rabbis have consistently interpreted the ominous word in Genesis 49:17 to mean that a false Messiah, or Antichrist, will come from the tribe of Dan. That could indeed be true. But Dan’s exclusion may also be the result of something else. You see, God specifically says in Deuteronomy 29 that any tribe involved in idolatry would be separated from the remaining tribes of Israel. This is exactly what happened to Dan. When they came into the Promised Land, the people of Dan were given coastal territory. But they weren’t happy. They soon migrated north in search of new land, finally settling above the Sea of Galilee in close proximity to the pagans. Because of their geographic location, Dan fell into idolatry.
Consequently, just as Deuteronomy 29:21 prophesied, Dan was blotted out as a tribe. But when the Lord comes back and establishes His kingdom in Israel, guess who is the first tribe given their allotment. Dan (Ezekiel 48:1–3). Such is the incredible grace, mercy, and forgiveness of our God. Jon Courson’s Application Commentary
So that's the last of what my commentaries say. I'm afraid we'll have to figure this out for ourselves. So, what strikes you as interesting, important, or possible or outright crazy so far?
Hope in Hard Times - The Final Curtain: Revelation 
That kind of muddies the waters a bit doesn't it? I don't know... there's still more, but I'll have to write about it tomorrow, God willing. 
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