I don't think you can hold more than 5 unless you purchase with real money. Its not even worth buying because I find candy all the time. Just clear out some enemies/boxes/bosses and after the time is up, a bush will appear. There is a chance that instead of a normal enemy you may find a candy monster which always drops 5 candies.
I have every monster in the game now, and 200 something coins (Good luck on pulling 5 coin boxes)
Is it a permanent raise, can anyone confirm or deny this?
If it's permanent I'll just buy the max amount...
I am unsure as it is so hard to find anything on this game but I'm going to assume that you probably get whatever amount you pay for but you are not able to go above (back to the same amount) then you already have. Also I believe 5 is always the permanent amount.
I can't confirm any of the aforementioned but I am still trying to figure out the official increment.