Vaal Oversoul dead, Apex gone, Quest not complete

Killed Vaal Oversoul solo on merc, corpse is still here on the ground, door is open but unable to pass through the door due to incomplete quest.

Assuming on inability to resummon Vaal if I was to let his room reset due to the lack of The Apex.

Any ideas what I could do to solve this?

You are missing part of the quest. Yes you can kill Vaal again after a reset.