If I were to create a community on Mbin, could Lemmy and Piefed users join?

Oh yes! Look for instance !AskMbin@fedia.io, I was able to post there from Lemmy

I believe so, at least theoretically...

I can access some magazines on Lemmy, such as !jazz@kbin.earth, though I don't see any posts on Lemm.ee... Could be a matter of time, it can take some time to federate. I think it should be possible, though.

Edit: I can confirm it is now federating! I can't see older posts, but since I subbed to the mag on Lemm.ee, I can see new activity on the mag.

Yep. Some responses may be delayed from being seen for a short time while the data federates, but everyone will be able to see & respond.

Does Mbin federate downvotes, yet?

They can, but they might have trouble discovering it. Other instances won't become aware of your magazine until one of their users searches for it explicitly, and even then posts won't be federated until someone subscribes. So you've got some advertisement to do.

This is a general problem though which Lemmy suffers from too. Lemmy has this problem less because there's third-party websites which list all Lemmy communities and you can browse them there. But those don't support Mbin magazines from what I've seen.

One way to overcome this problem that I know of is Lemmy Federate. As I understand it, it tries to subscribe to your magazine from various instances (including big ones) using its own account, so it starts getting federated there.