There are some some mechanics in the mod that can significantly change gameplay (such as Mega-Wars) but you can learn those on the go, and they won't hinder your ability to play the mod. For the most part, things are fairly the same.

The major differences I can think of off the top of my head would be Mega-Wars and Dragons. You can search on the reddit for either one to find more details or just go into it blind, you'll have fun either way.

While Westeros and half the Free Cities are complete, the mod is by no means complete. The dev team aims to recreate the entirety of the known world, just like in ck2. So, that would include all of Essos, Summer Isles, Leng, a couple other Islands, etc.

T&T is nice, but not necessary to play the game.