I change from my regular clothes into my undies and drysuit standing by the side of the truck behind the door, depending on the divesite and how many non-divers are around. I try to be discrete if there are people around that I don't know, but it's hard to be discreet with anything that involves a p-valve. I think that I broke Wisconsin state laws once when I briefly exposed myself to a woman I didn't notice who was sitting across the road. It does get pretty cold stripping down in the elements, but I don't stand around and chat while I'm doing that.
Most women I know who dive dry manage to change very discreetly since I've never seen them running around in their skivies.
They probably put their undies on before or in the car/dive vehicle. For the modest diver, there are towels or little changing tents that collapse flat.
There is a cave in Missouri that I've been to that is in a quiet park. At least the park is quiet until I start to strip down or attach the p-valve. Then it seems that the whole town parades by to chat us up and see what is going on. I guess they misunderstood the idea of the 7' hose.
As for the mask freezing, just take it off as soon as you can after you exit. I've had water freeze on my gear before. It will usually melt in the water, but can make getting geared up harder for the second dive, although most people stick to one dive when it's that cold.
Rinsing the drysuit? I don't do that- simply rinse the dirt off the feet right after I get out of it and then hang/lay it out to dry. Then again, I dive fresh, so salt is not an issue. I don't think that there would be anything wrong with using municipal tap water to rinse your suit, but I'm not a rinse expert.