The high mach number option is under general settings > fluids > flow characteristics (at the bottom). As for the other two, I don't know sorry.
#1 see Owen's note on where to find high mach option - if you do not expect high mach numbers then look at your simulation as to where the problem is and increase mesh resolution locally
#2 a vortex crosses the pressure opening means that at one of your pressure inlet BCs you have flow both in and out. The best solution is to increase the length of leadin (i.e. if it is the end of a pipe make that pipe longer so that it is a uniform flow in the same direction).
#3 have you taken a cross-section plot in the 3 main planes to look for extremely low temperatures? I've never used humidity but it may simply be complaining that you have temperatures below your dewpoint and because it does not do phase changes it considers it an error.