Ok, so act III completed before I had time to visit 'Ascend to madness', the dungeon seems to be closed now... is there still a way to visit the place? Or am I stuck at 1 point from the title?
Unfortunately, no, there is no way to complete the dungeon now, and you are indeed stuck 1 point from the title. Arenanet posted the following yesterday, bold mine:
Seeing as Phase 4 has now begun, the dungeon is closed and there is no way for you to complete progress on that title, which will also unfortunately block you from completing the Emissary of the Mad King title. Sorry to be the bearer of bad news!