Xaliasa hasn't been reborn to summon them!
*Facepalm* Totally forgot the runewell activation doesn't just cause the sinkhole but also revives him! Definitely wasn't prepared for him to earthglide the elemental down, so I was completely winging it.
Really like your idea of the hounds guarding the bones (could also play off the howl as a ghostly echo/ward/haunt effect, since he never saw the hounds but just fled from the sound). Added bonus - my players are paranoid about the waters in the ruins after the waters of lamashtu and the runewell summoning sinspawn, so they'll likely not disturb them if I just let them explore a bit and mention the skeleton in the water as part of a perception check.
The revelation quill surviving could be a great tool for getting them back to Sandpoint after Mokmurian too - it could begin scribbling madly in Thassilonian upon Xaliasa's awakening, maybe with the same unsettling handwriting they saw on the walls of the meditation chamber....lots of room to play there!
Thanks so much!! You turned the disaster in my head into a very cool opportunity!