Text:
Lord, House Umber Melee. Challenges: Kneel Greatjon Umber to have him participate in the current challenge as a defender. Then, if your Title supports the defending players's Title, Greatjon Umber claims 1 power.

Question:

If my Greatjon is already participating in a challenge as a Defender, standing, and I use his ability to have him participate (again) as a defender, does the "Then..." effect triggers and resolves successfully?

In other words, did his ability first part resolve successfully?

No.

A character is either participating in a challenge or it isn't. So having a character that is already participating in a challenge participate again would be treated the same as kneeling a character that is already kneeling. That is, even if it is legal to trigger the effect, but it does not resolve successfully - which means any dependent effect (like anything after an associated "then") will not happen.

(This is also why a standing Naval character that has been removed can be declared again, but a standing Naval character that is still participating cannot.)

So sad, that's what I expected :-(

Thanks a lot for your answer!