This proposal doesn’t say anything about the creation of categories (your item #3). A category must exist before a moderator can be assigned.
Do not confuse categories (collections of topics) with groups (collections of users). A user cannot join a category, only a group. Category access can be limited to particular groups. I’m addressing non-admin group invitations elsewhere (see https://meta.discourse.org/t/invitations-to-group-restricted-topics/25895).
Short answer: All this stuff must currently go through admin.