Re: What are the requirements for the joining the Federation?
Even the first of those two doesn't really appear to be that strict a requirement. The point in the episode "Attached" was that on Kesprytt III, one part of the planet, under the so-called Kes government, would join the Federation while the other, under Prytt government, would not. It would suffice for the joining part to have a single government, then - the Kes would not be required to conquer the entire planet first. Had this taken place, it would have been a first, but it still could have happened.
The situation on Kesprytt III was treated as an atypical case to be evaluated by our heroes and Picard made a point of it being not the norm:
"Every member of the Federation entered as a unified world, and that unity said something about them. That they had resolved certain social and political differences and they were now ready to become part a larger community..."
And ultimately as the episode panned out, the disharmony & animosity between the Kes and the Prytt led to a recommendation by our heroes that the planet wasn't ready for Federation membership at all.
"Shout, shout, let it all out..."