When a character or upgrade or support mentions "this die" its referring specifically to the die that card brings correct?

I've seen a lot of people use Kanan's ability to resolve Force Speed, effectively getting 3 actions out of it since you didnt use an action to resolve it in the first place. Kanan specifies "one of this die" though, which would indicate only his character dice.

Yes, "this die" is the die from the card. Kanan's ability works on his own dice only.

What about a relby being brought on with ace and you have a relby in play?

Relby (or any other card) with Ace in the Hole can't do action from the card because that card is not in play. Only dice inherent abilities works with Ace in the Hole. (RR: Inherent dice abilities other than specials never use the words “before” or “after” or “while.”)