Explore Mad King's world after act III

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:

We want to give you a heads up about Phase 4 of the Halloween event. Phase 4 will start at 9:00 AM PDT/16:00 GMT on October 31, and will run until we publish a new build on November 1 (which will be sometime after Noon PDT/19:00 GMT).

Once Phase 4 starts, the Ascent to Madness dungeon will no longer be available – but you will still be able to enter the Mad King’s realm, including the Labyrinth, the Clocktower, Reaper’s Rumble and Lunatic Inquisition.

-Source

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!