Consider how a user who pays the invoice logs in, would he/she log into their subdomain or would they log in the public site? Ideally they would be able to only log into the public site with same credentials as the subdomain site to pay invoice.
(not possible)
Registering an account creates an org and subscriber in the public schema and creates a user in the org's prefixed schema, where subscriber belongs to org and user.
Not possible because can't have dynamic prefixed migrations with each user created.