What each user tells your agent stays with them, whichever model answers.
Memory is rolling out now. It stays off until you switch it on for a key, and there is nothing to sign up for.
Not yet. It is rolling out now, and the docs and pricing go up when it reaches keys. There is no list to join.
No. It is off by default, and you opt in per key, per request.
Your agent keeps its brain. Flux gives it the rest, on one key and one bill.