bad acting

the scene with the priests discussing who should perform the exorcist was awful

It was fine acting, and it was one priest and one bishop talking, with the bishop suggesting to Karras that it might be better to get someone with experience - and Karras's face briefly falls with the sudden realization that at this point the bishop does not think Karras is the man to perform the exorcism. More wonderful, subtle acting by Jason Miller.

The last scene's problem wasn't with the acting, but with the dialogue, which it seems most audiences are to thick to "get" that it's an extenuation of the goofy-mythical movie chat between Kinderman and Karras earlier in the film, implying that just as Kinderman and Karras were enjoying a new-found friendship, so too were Kinderman and Dyer.

I'm a huge fan of playwright, Jason Miller, so I'll have to disagree with the OP.

You obviously have no idea what bad acting is, as there is simply no bad acting in this film.
Unless, you are talking about "The Version You've Never Seen"
Which in my opinion, should remain unseen by everyone.

The acting was fine. They weren't professionals, but they didn't need to be for the brief few minutes of their scene. The scene at the end was well handled by Cobb; being one of the more underrated actors of his time.

speaking of "bad" acting, I thought Burstyn's reaction to the planchette moving away from her hand was fairly wooden. That happens to me, I'm running out of the basement with my pockets empty.

And good acting is what? Mark Hamill in Star Wars?

I'm confused. I don't remember any scene at the end with Kinderman and Dyer. From what I remember the ending scene I just Dyer standing there alone.