I believe that's a USA peculiarity. Where I live, the usual residential and commercial wiring is from 13.8 kV to 127V/220V through a three-phase delta-wye transformer, in which the primary connects between each pair of phases, and the secondary connects between one phase and the neutral (the high-voltage primary side does not have a neutral). When one phase of the high voltage side is lost (very common, since each high-voltage phase is a separate wire and has an independent fuse upstream of the transformer), what happens is that one phase of the low voltage side stays normal (the one between the two intact high voltage phases), and the other two have a lower voltage which varies depending on their relative load.
Yep. Sounds like you have true 3-phase service, whereas most places in the US just get split-phase.
All depends on how it failed.
I don't have a power outage more than once a year, but we manage to blow a breaker more than a few times a decade (vacuum + water boiler was one).
On a commercial 3 phase service, yes, connect redundant PSUs to separate phases, since each phase on the panel actually corresponds to each phase of the grid.