assuming constant speed, not IAS
which sort of speed are you keeping constant, here ? If EAS, then the loads would stay the same, if IAS (or CAS - near enough) they would reduce a tad, TAS and groundspeed are not really relevant ...
you could go faster and keep your arm out of the window as the air pressure reduced
that all depends on what you are doing with your EAS
super sonic air related
not quite ... compressibility effects will be measurable well before you get to sonic velocities.
It's no different to most aircraft limitations ... the pilot sees the limiting case (of a number of various limits) either in the POH/AFM or on his/her gauges. With the barberpole, the Vmo limit is limiting (ie Vmo<Mmo, measured as IAS) at lower levels while Mmo becomes limiting (ie Mmo<Vmo, measured as IAS) at higher levels.
the air is still passing around and over you at 320kts ... even though the mach reading is .99999
the speeds are related by the various equations (you probably don't want to go there). Typically, the Mmo might be mid- to high-0.8s (doesn't really matter .. it is a Type-specific thing). You would not see an Mmo near sonic velocity as the general idea is to operate reasonably subsonic (say 0.9) or supersonic (say 1.2). Sustained operation in the transonic region is not a useful idea as the drag problems don't offer any advantage.