You don't create coinjoins with yourself, other people can use your coordinator.
Yes, they can. But 'can' isn't the same as 'will', so they must probably will not because they will stick with the default one, which is zkSNACKs. You aren't advertising the negatives of zkSNACKs and who they are in bed with, and people not reading this discussion and similar ones wouldn't know why they should abandon zkSNACKs
People are not stuck using the zkSNACKs coordinator.
Not in the literal sense of the world, obviously. Check the reply above.