Here's my take on what I would do if I were the GA in a situation like that and you were able to hold the flight until T-2 instead of T-15 (I know it's easy to play Monday Quarterback):

If nobody else was on the UG standby list: tell the 2 guys to take their new seats & the GA clear it with the FA.

If others were on the standby list: Upgrade the next in line at T-15 and hold the flight as long as possible. If the other guys show up, tell them that you did them a favor & held the plane, but at T-15 you had to upgrade others, assuming they aren't going to show up. They'd then have the choice of taking available seats or waiting for the next flight (no compensation since it was an ATC delay). They guys might complain that you "gave away" their FC seats, but if they are rude, but a simple explanation that favors were done to keep the plane an extra 12 minutes might help.