3) When #2 happens, my units who are supply-cutoff are COMPLETELY IMMOBILE AND USELESS. It seems like this shouldn't happen IMMEDIATELY, as I then have no way at all of extracting them by "backing away from the front" and must instead either write them off as dead or expend a lot of my other forces in trying to retrieve them. I wouldn't mind the latter if I had some capability to self-extract them first (understanding it would be very limited).
I'll let some other answer more fully, but when units are cut off from supply, they dont lose (at least they shouldnt be loosing all mobility immeditately). Unit can "carry" more supply than they use, so they should be able to maintain readiness for a while. I watch the Unit's supply level (stock, in, request mainly), to check supply status. if stock is 0, then I try to improve my supply, since I know I could get into trouble fast if cut off.) If I understand it correctly, in the unit in stock slot, the number in parentheses is the number of turns a unit can last without supply coming in.).
Also, check the right-hand part of the unit panel aand make sure supply is set to 100%. Apparently some scenarios have some units wehre that is set to 0, so units don't request full supply.