Thornwatch - How does paying for Hero Actions work?
There are two types of powers - tracker powers and action cards. In order to use either you must pay for them either with slotted cards or discarded cards. See page 14 / 15 of the rules.


If you have stacked power cards under an Action Cards, then when you activate that Action Card the Action Card and all the stacked cards that are powering it are discarded. You do not pay a second time with cards from your hand.
When using an Action Card that you had slotted on your Tracker, it and all stacked cards powering it are discarded. You don't get to keep the Action Card to re-use again later.
Yes you can slot, power then use an Action card in one turn, on the understanding that you can only Act once a turn. In this case I don't think there is a difference between stacking cards to power the Action Card or simply discarding straight from your hand to power the Action Card.