1.) Anyone find a way to set the difficulty for the USA campaign in RTI especially the last episode that says United States?
You can change the battle difficulty in the in-game options, but I don't think you can change the campaign difficulty.

2.) Is the USA only playable in RTI where it says United States or can you select USA after selecting Grand Campaign from the main menu after some unlock?
USA is only playable in RTI, with mods it may be possible to start a Grand Campaign as the 13 Colonies.

3.) Does the game force you to stop playing at a certain date in either the Grand Campaign or RTI USA mini Campaign? Or can you play untill you actually have world domination no matter how many turns it takes?
You can continue playing as long as you want. Other countries have a huge head start though (obviously). I won chapter 4 and clicked continue so I could try to conquer the world later on, but after a turn saved it and started my first Grand Campaign. Maybe after a few Grand Campaigns I'll come back to it.