I don't see this in the RRG or under the FAQ's. I know the RRG says that once a character is bypassed by stealth, they cannot be declared as a defender. However, if I use--say--Areo Hotah (Core) and remove the attacking character who stealthed one of my characters from the challenge, can I now use my character who was stealthed as a defender?

No.

However if you have a card effect that says to add them to the challenge you could use that. (Wardens of the North, Winterfell Kennel Master, Nymeria (FotOG))