Did they change the difficulty requirements?

In the previous versions of the game, you could switch to higher difficulties after you beat the last cthonic boss.

Did they change it so that you now have to complete the whole malmouth arc before you can bump the difficulty?

No, you still unlock the next difficulty after killing the Loghorrean.

Awesome. Thank you.

Is it me, or does finishing the expansion content overlevel you for Epic?

I was 52 after beating act 5 boss. Was level 37 after beating log and unlocking elite. If you do absolutely everything then yes you will be over leveled, but you can make up for it by skipping act 5 in elite if it bothers you.

not really the enemies scale extremely well in elite mobs will catch up by act 2 then in act 5 they will probably out lvl you by around 3-4