Re the FL250 restriction with only one a/c pack operating...
I always understood that the altitude restriction was to allow the airplane to descend to 10,000 feet in the case of the other a/c pack failing. (in 2 a/c pack airplanes)
At 'normal' cruise altitudes, the time required to descend verses the cabin altitude climb rate is the concern.
Of course, this is still a concern if the initial a/c pack failed just after takeoff, but then complicated by the fuel usage rate at the lower level.
Cheers...FD...
