With limited overhead storage space in the small aircraft type u said it was, I would think many people would prefer to be on the plane and be sure they didn't have to check their carry-on bag, versus boarding last to avoid having to disembark if needed as a vol. I realize that's not your preference but definitely would be my preference to do it the way she timed it

Sounds like she was trying to make light of needing you to volunteer after all. Had you spoken with the gate agent at all before boarding to see how likely you'd be needed as a volunteer? One can always turn down being a volunteer even though one's clicked in the app for a specific dollar amount bid. That stinks about them messing up your flight. Was it a location where you were volunteering where there was an admirals club? cuz I certainly would have just taken care of it there if it was going to take so long to fix it.