1) It's fine to solo, it's not a punishing survival experience. It's more fun with coop but that isn't a deal breaker. I do both.
2) Most of the newer content (Mistlands / etc) will be many hours into the game so other than magic, it won' t be terribly different. It's up to you. Test branches often come with numerous bugs.

---I'm trying to wait for the proper build but I may not hold out either.