What more can he do, other than get canned, to be accountable?

Nothing, of course - that's why he is not accountable. Because of the way things currently work, he was not in a situation to be able to be accountable.

Maybe we can change that eventually.

Take a pay cut, push back on investors/board, wait until after Christmas...

Apologise and donate his fortune to homeless charities?

One option is to pay back enough of his compensation from that period (and money made off of investments funded by that compensation) to make his total comp over his tenure equal to that of a low-to-mid-level line worker.

Some say that money is the unit of caring. If that's true, then nothing expresses an executive's commitment to accountability more than him forfeiting enough of his total compensation to put himself nearly at the bottom of the totem pole.