I'd forgotten that bit; on reflection I think you're right, though it is a bit too convenient.

Mind you, I'm not sure how Ransom knows he's dealing with a zombie/ed corpse - the swallowing broken glass bit might merely prove that the Forces of Evil don't care what state Weston's body winds up in as it's not their long term problem.

I also, from a theological standpoint, see no reason why you can't exorcise a possessed corpse, and have been worth at least giving it a go. Though I suppose since Ransom is convinced God wants him to deal with the problem on a purely physical plane, it's plausible that he wouldn't.

I do think that aspect of the plot's a mess, though.