Plans

Almost always because it asks several things at once or has no answerable form. Closed questions are editable and reopen automatically once a moderator or three high-reputation members agree the edit fixed it.

You gain reputation when an answer is upvoted and considerably more when it is accepted by the asker. You cannot gain reputation from self-accepted answers, and reputation is never purchasable.

Bounties move reputation from your account to whoever answers a question that has gone quiet. They are the main tool for unsticking hard questions and they cannot be refunded once placed.

Not once it has an accepted answer. The archive is the point of the site, and deleting a solved thread takes value from everyone who would have found it later.

Only if you disclose it and have verified the answer yourself. Unverified generated answers are removed on sight, because an answer nobody checked is worse than no answer.