Honestly the only way it makes sense to me is if most of what Eragon was told about Galbatorix's rise was wrong. If Galbatorix had popular support among younger Riders and Wild Dragons, massive numbers, and his victory over Vroengard was inevitable.

Then, rather than accept that change was gonna happen and they may as well be on the right side of history, the elven Riders turned Vroengard into a trap for Galbatorix's forces. The 13 Forsworn and Galbatorix were the only survivors of the detonation, and the reason two Forsworn are recorded as "dying" in that battle is that they were on the Riders' side until after the detonation so the Elven government found it more expedient to pretend they were dead.

Then the elves retreated because they genuinely truly believed they could wipe out all the survivors on Galbatorix's side with zero losses when their invincible army rolled out. The only complete failure of courage was Islanzadi's, but since she was the monarch then only her courage had to fail. And over the past hundred years she's more or less succeeded at convincing her people that Galbatorix is this invincible mastermind master of all magic or something while Brom is, separately but not completely separately, assassinating every single Forsworn instead of the one or two who killed OG Saphira