Sky, I would say differently regarding the 2 last heads. I'm not trying to be contentious, but if it is, I apologize
In Revelation 17:10, one head "is". That is, when it is written, the 6th head exists. Since this is during the time that the Papal beast (5th head) is out of action (the beast that "was" and "is not"), and America existed and was established by this point, America can fulfill this role well.
But another reason is because of how Atheist France is referred to in Revelation 11:13. It is referred to as "the tenth part of the city". Which city? The one referred to in Revelation 11:8. I won't go into details unless you ask, but that "great city" and the "beast that ascendeth out of the bottomless pit" is Babylon. That is, the prophecy classifies Atheistic France as a subunit of Babylon, particularly the Papal beast.
The point? In the prophecy, Atheistic France is not a world power. It is part of a world power. It is 1 out of 10 horns. America, on the other hand, gets its own beast and plays a very dominant role in the end times. If God had to pick an empire to represent as a head with the likes of Babylon, Medo-Persia, Greece, Rome, and the Papacy, America is the much more logical pick.
I would peg the last head as the united 10 kingdoms found in Revelation 17. Here's why:
1.) In Revelation 17:12, the angel informs John that the 10 horns have no kingdom yet. This implies that they will eventually have a kingdom. The passage calls them kings, and it is fitting that kings should eventually have a kingdom. Revelation 17:17 echoes this, specifically saying that they have kingdoms to give to the beast. This makes them a potential target for the 7th head.
2.) Revelation 17:10 says that the last one "must continue a short space". This indicates time. Between Apostate Protestantism and the 10 horns, the 10 horns is promised a short space of time to reign with the beast. Revelation 17:12 says that it reigns for 1 hour. As the "image of the beast", Apostate Protestantism does not have a set time to reign. This favors the 10 horns.
3.) In Revelation 13:11, it says that the second beast "speaks like a dragon". When does this happen? When the "image of the beast" is set up in Revelation 13:15. "...the image of the beast should both speak..." It treats the second beast and the "image of the beast" as one entity when it is first introduced in Revelation 13:11 by saying right off the bat that the second beast "spake as a dragon".
Compare this to Revelation 13:2, when Rome gave the Papacy its power, and Revelation 13:5, when Rome gave the Papacy its ability to "speak". But here, two separate entities come together. One beast gives its power to another.
The short version of point 3 is this: Rome and the Papacy are counted as 2 heads because they are treated as separate beasts. The second beast and the "image of the beast", however, are treated as a single beast. America and Apostate Protestantism overlap, allowing for the 10 kings to have a spot.
A lot to swallow at once, I know. But if you have objections, let me know. The truth loses nothing by inquiry.