Personally, I think spending miles for the sole sake of the fear of an airline going bankrupt and/or going out of business is spending miles unwisely. I beg to differ with your strategy. My advise is to hold on to your miles. IF United does go under, you will have ample time to transfer your miles to another carrier (ie. converting MP miles to Diners Club Rewards and then transferring them over to AAdvantage or Worldperks, etc). But don't burn/waste your Mileage Plus miles because of pure speculation and fear. You will probably end up regretting making that decision once United gets back on its feet. If you do use up your miles, do it because you really want to go somewhere or purchase something.
I know how you feel! I've had similar experiences in the past. Unfortunately UAL Mileage Plus reps are probably some of the lesser trained out there (bad leadership = bad training) and the ones who know their stuff the least, compared to other FF program reps. However, I usually give them a break because United's MP program is pretty complicated and does take time to fully comprehend due to it's multi-level structure. I hate to say this but the best way to get around that and the best strategy that's worked for most people is to politely hang up and then call back. Keep doing it until you get your desired response... or at least a decent explanation of why your request can't be processed.
It's worked for me (particularly with discrepancies in claiming bonus miles due to a United error).
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!