The Tavern

I think that you should not need to travel back to a discovered altar to join its god. Once you have found an altar, you should be able to convert to it, no matter where you are. Having to travel back doesn't add any fun, and will become even more pointless without monster respawn.
Changing god can be a X ability option instead.

This makes me curious how joining a god is handled in Hellcrawl

Same as regular crawl except you press p instead of >. There are not many opportunities to switch gods lategame aside from the slime altar or converting to Lugonu in abyss at the moment.

Hmm. This would make Abyss and Slime Pits easier - you could go in and only convert if you hit trouble. Maybe at significant turn cost, like memorizing a spell?