How can I make and farm Demon Altars in a world generated without one?

Demon Altars can't be created, either by crafting or by enemy drops. They are purely from world generation. I don't recall a version of Terraria that didn't have Demon Altars, but if for whatever reason your world doesn't have any, you're out of luck. You'll have to create a new world to spawn them.

Side note: As for the guide respawning, make sure the new guide has a valid house to move into. Also, it can take a day or two (or three) before he actually respawns, it's kind of random. There might not be a bug at all, you just have to be patient.