Not sure what you mean about the Ur Dragon after beating the game, you mean NG+ and such? Everything is the same. The only place which becomes more difficult as you finish it is the expansion content, since BBI becomes a lot harder after clearing it. As for how tough he is in general, he's not really hard if you're prepared but he can be quite intense the first few times. Main annoyance is the first times you don't have the dmg to kill him fast enough, and he flies off after 10mins, then you need to go back, sleep and come back again. He doesn't reset it's just annoying, but at the same time you can restock on consumables and shit. Evil Eyes aren't invincible, and the first room should always be an Evil Eye room for any level of the Everfall and until you kill it the rest is locked, but it might be buggy I don't know. You shouldn't be able to do Everfall stuff until you kill the Eye.
As for DA stuff accordion, it's noticeable mostly when you go there. I don't know if they patched the main game to parity with the expansion or if the expansion changes only come if you have DA, but there's quite a few changes to the old world too. Some mobs are replaced by new versions, a lot of skills and items were rebalanced and you get a lot more port crystals in the first playthrough, like you get 4 or 5 now I believe, while originally you only got one in the blue moon tower which was really late. Now you get one in like witchwood, shadowfort, bluemoon and great wall or something. So travelling is a lot less annoying.