When you feel the ground rumble, know an OGRE is coming.
1) Do Mjolnir or the shield count as Basic Action cards or Character cards?
Character Cards, hence the character type dice
2) Do the three dice one each basic action card count against the twenty maximum of character card dice?
No, that is besides your 20 CHARACTER dice
3) up to eight character cards: can I use as few as one and as many as eight or do I have to have eight? And as little as one and up to twenty dice split amongst those cards?
I believe you can use as many as 8, meaning 8 is a limit. If you want to use as little as 1 than sure. But keep in mind each card has a dice limit as well. I am not completely 100% sure of this I would need to take another look at the rules.