Yes. You can continue recruiting creators until the campaign is closed.
There is no specific privacy setting, but you could restrict campaigns to certain creators or groups and send out invites to those creators or groups.
Yes, they receive an email and in-app rejection notification.
There isn’t anything different about the creator program payouts from typical partner commissions. Task payouts will appear as pending actions according to the immediate locking and clearing schedule defined in the contract.
Creators can generate tracking links for campaign-related content from their task manager as part of the task and track performance like clicks and actions against it.
No, you aren't required to use tracking links. impact.com gets social insights per post like shares, likes, engagements, comments, etc. without using links. You'll need to use links or promo codes to track performance. It is possible to track performance even if there is no associated payout for the campaign. So, if you're doing flat fees only you could still use links to track performance.
Yes, if you have a sale event type you get the same functionality as other affiliate programs (though still restricted by edition). However, you can't set up payout groups and the like for PayPerTask event types.
Creators are not paid automatically when they complete their tasks. Payments are only generated when you click the Pay button on a per-creator basis - the exact payment date after that is determined by the locking and clearing schedule you have specified in your template terms.