You sound like me when I was 16 and thought that all mechanics were theives, not trusting anyone. So..................I decided to learn how to fix my own car myself, as much as possible using a store bought manual and K-mart tools and later eventually became a mechanic myself.
I now know how and why people like you become upset as you're not getting the full story IMHO. I think that what's happening is that there's a comunication loss between you and those who are actually trying to repair your car.
Now, I also work at a dealership and I've seen the problem you're experiencing. The trans siezes up where it goes through the glass and although it can be freed up, lubed, sometimes a replacement is a better option depending on the amount of corrosion. If the trans is seized and one tries to work the motor too long, it doesn't just idle freely as you think, it's stuck (can't move) and it will overheat and burn up if left on long enough. I have yet to see a reason why anyone would ever take an electric wiper motor apart as they couldn't fix it, especially if overheated and with a bad wiper trans SO I think that you've misunderstood what's being told to you and possibly that the service writer doesn't have the right translation of the facts from the tech (happens sometimes). It's more likely that the motor was seperated from the trans instead to see which part was seized, not dissassembled.
Another problem you're getting into here is that you're going between two repair shops and believe it or not, there are times when two different techs will have two different opinions, one being more right than the other. One probably has actual experience with your problem while the other doesn't. What you need to do is just decide to have one place fix it and be done with it. Don't assume that anyone's deliberately lying to you as that's probably not true. All Volvo dealer personelle are rated by Volvo and sometimes an independant CSI group as well SO it's very important to them to try their best to do a good job, not upset their customers. If you really think that your dealer people are not treating you right, try talking nicely to the service manager as it's to his best interest to get your car fixed right the first time, even if you presently don't seem to think so. Don't go in there with an "attitude" as that will almost never help you in the auto repair world anywhere (or anywhere else for that matter).
Another idea, just leave it unfixed. If one really thinks about it, do you really NEED a rear wiper? Although my old wagon has one and it's still working, I wouldn't pay alot of $$ for it if it ever broke someday as I rarely use it and could live w/o it.