Latitude - could be that the CO2 snow cover in the southern winter is interfering, but I just don't know

K from several sources. Certainly K-fspar in rocks and dusts. May also be evaporitic. No evidence. Don't know

K in highlands you point out is just north of the alleged "Maxwell" lakes. May or may not be relevant. Don't know

As for dark streaks, I've looked into the topic. The recent publicity surrounds very lightweight work from a "bimbo". Much better science has already been done - those who did the work do not argue for flow of liquid water causing the streaks (for one thing, within a few days it should freeze or dry out, so colour should revert to normal).

Some sort of dry dust flow is likely, but what is the trigger? Marsquakes from a slumbering Olympus Mons? Changes from ice to vapour letting things move?
Again, - Don't know.

Some lab experiments to simulate these processes could be very instructive.