Can I create a community on an instance with which I don’t have an account?

No.

Can I serve as a moderator for a community with an account on an instance different than the community?

Yes, provided an existing moderator (or the community-creator who is an automatic mod) adds you.

The traditional way to create an community on an instance where you don’t have an account is to pair with someone who DOES have an account there and have them create it for you and hand-off moderating to you once created. There’s very little reason to create a community on a remote instance though, unless you want to slot in with a topic-focused instance where you don’t have an account (like if I wanted to create a new pathfinder sub I’d probably consider asking the admins at pathfinder.social to create it and add me just so it’s easily discovered on an instance that’s exclusively dedicated to pathfinder topics… but for general communities I’d just make them on lemmy.world where my account is).