Obviously in real life most people (certainly, I think, most LWers) are VERY VERY HIGH above the "zero line" or whatever, so these sorts of questions feel pretty abstract to me.

I don't think that's obvious. Something like 1/5 of LW is depressed.

I suspect commuting is below the zero line, i.e. people would fast-forward through their commutes even if it meant never getting those hours back.