If you make a "multiple" Action (like a Rapid Fire) and spend 1 Fortune (or get an Assist) to roll an extra White Die, do you get this dice for evey roll of the "multiple" action or only for one?

Not sure it is clarified in the rules and probably worth a rules question to Jay for an official response.

I keep changing my mind about which way I would fall, so i can't even give an opinion!

Each roll is separate. The fortune points section says you add dice to an action check (not to an action), and you add them when creating the pool (and you create a new pool for each roll). So, each check is separate.

Also, multi-action powers are always the strongest options, and you really don't need to buff them even further.

Dice are added to a single pool/check at the time the pool is created.