The official answer is as above - you can only take 1 action while you have the card, and that action can be to only remove the card. Once you've done that, you no longer have the card to abide by the 'only take 1 action' part, so are free to take a second action. Essentially, it comes down to what point in the heroes turn are the number of actions you get determined - at the beginning (in which case, stun WOULD mean you only have one action in a turn, period) or continually (meaning once you get rid of it, you're free to take your second action)… My belief is the latter. Make sense?
This is where a lot of the confusion is coming from - the card doesn't say you can "only take 1 action" - that's just the net effect. It simply says you have to spend your action in a certain way. I'd be willing to wager that they carefully avoided using that wording to prevent just such a misunderstanding.
The card says "Spend 1 action to get rid of this card; this is the only action you can perform during your turn while you have this card." As long as you don't read more than that into the effect, it's pretty straightforward.
- At the start of the stunned hero's turn, they process any start of turn effects and refresh any exhausted cards as usual.
- Then the hero gets 2 actions. The Stun card does not state that the hero loses or gains any actions on their turn, so this is unaffected.
- For the first action, the hero has a Stun card, so only 1 choice is available - the hero must get rid of the stun.
- Now the hero has 1 action remaining on their turn, and with no Stun card they are free to spend that action however they choose.
It doesn't really matter when the number of actions is determined during a turn, since the Stun card does not change the number of actions available to the player. Rather than modifying the number of actions the hero gets, the effect simply causes the hero to waste an action clearing the status effect. The end result is the same - the hero (or monster) loses 1 action.