Once you hit the gong (after completing the council quests), it is a one way trip to the final area of the game. You can safely hit the gong now, and your character will simply make a comment (or maybe that is just before killing the succubus).
However, from where you are in the game now, there are still a series of quests to complete (talk to Zandalor), and you still have access to Rivertown, Verdistis and the Dark Forest (so you will be able to complete everything else). Since you have killed the succubus to clear the council hall, you can find the imp that 'borrowed' your teleporter stones, and the teleporter pad will now work.
BTW, there is a way to return from the wastelands by
hex teleporting. However, I do not know if this can be done to complete quests. The quest log is cleared when you enter the wastelands, which may only be cosmetic or quest information could actually be removed.
