Hrm with Doom disabled for Nahuatl that would make getting the reforms as easy as getting 5 vassals repeatedly. How does a Mayan High American Empire that starts with 20+ provinces that only have their religion and culture work? They can't release those provinces as a separate nation so do they just lose the cores and keep the land and have to re-core, or do they completely give up the land to nobody?
Mayan works the same with Primitives and Non-Primitives, you WILL lose the land and someone will get that land it doesnt matter if you are the only tag that has cores on all your land, you are losing some of it and a different tag will get it(with cores)
Nahuatl is "hard" because if you start in the Americas there is no one to vassalage you need vassals not Protectorates therefore you gotta go to Europe for that, but yes is fairly trivial.
Inti might be even more Trivial if you are able to earn "invisible" Authority because once is 100 you can pass reforms as soon as you have positive stab and no rebels.(They might also be impossible in Ironman if Authority can not be gain besides the console, I am not playing with them enough to test it out)
EDIT: I just tested it, as High American Inti you DO earn invisible Authority meaning you can easily pass reforms once you reach 100 and you will not lose this Authority by passing reforms, I must also point out that you also get the benefits of Authority(lower Unrest and Stability discount) so is a double plus for Non-Primitive Inti, by far the easiest of the 3 to reform if you start as a Non-Primitive.
EDIT2: As High American Inti you do NOT lose reforms even if rebels break your country, this seems fair to me considering that Nahuatl doesnt even have to worry about Doom at all(So they can't lose Reforms either)