1) Yes.
2) All. Sometimes we flub and spend a few points and say "Oh man, I shoulda...". I use this rule myself in TT games and the rule is simple: "Proivding the character has not made use of the ability or power, then it may be bought back." The moment you use it however it's bought and paid for.
3) Only the most recent expediture for EXP. As far as Backgrounds go, well, if you lose your job or get kicked out of the Teragen, those things happen and you should not be 'rewarded EXP' be cause you decided to quit being an Elite (Backing) and start your own religion (Followers).
Everyone should be keeping Track of EXP and only the most recent spending should be permitted to be negated providing the person has not publically used those powers (or abilities) in a fiction.
Example: Balm uses Telepathy on Caramel to find out what really happend between her and Gabril Law on their trip to Rome. Later, Balm decides to mulligan her EXP but can't because she bought Telepathy and has used against someone. Buying it back causes serious continuity problems. If she raised other abilities or attributes during that time, she may buy those back, but she is now stuck with Telepathy.
4) Considering this could effect continuity, I vote for once every 6 months. You get 2 mulligans a year people, that's it.
Personal Note: As a penalty a person should NOT get back all of the EXP. A penalty of 1/3rd (rounded up) the EXP spent should be 'burned', permanently gone.
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Too hot to handle, too cold to hold!