So I just got the girl involved in the ritual

Spoilers

and the priest is already dead. so no one wants to examine it. I also can't find tinthian but am more worried that this has to be done chronologically. Correct me if I'm wrong.

there is actually no need to even pickup the girl as far as I know (I'm at the start of act 3), not sure who you are talking to about a dead priest, but if you mean the bishop, he should have talked to you a bit before disapearing...
and for tinthian, I also couldn't find him, until I scoured all the map I had access to completely.
I'll give you a hint though, he's having a bbq in rogue settlement... that won't unstuck you though, as tinthian doesn't become really usefull until later. (you can still do a little quest with him now, and have help for a testimony).
What I think you need though right now, is to rethink as to what the dead bishop told you when you last saw him, and I do mean the very last time.