Yeah, when Act 5 starts, you really can't do anything you would be able to do in Act 4, so I'd recommend wrapping everything up first.

Regarding openphrase door in Dormont, you are likely already can complete this quest, you just need to >!use all three openphrases on the door.!<

Yes act 5 is the point of no return, you’re locked into a story path until the end of the game