Always interesting to hear ideas - no bash here just some follow up questions for ya.
Why have pelts cooling the water if you're cooling the water with your phase change system? or do you mean pelts on your cpu / gpu and you're cooling the hot side of the pelt with water, which in turn is cooled by the phase change chiller - this is a workable design, sort of a hybrid cascade system
As far as condensation goes you're going to have it at the temps you want to go to - your two options are to A) insulate every cold surface in the system with closed cell foam / silicone / conformal coating etc B) make the case airtight and dessicate the air inside with a dehumidifier or chemical dessicant
Referring to the size aspect - you want to fit the whole phase change system in a 20mm box? I have to assume that 20mm was a typo - that is *really* small. Your average small hermetic compressor is going to be around 200mm high and equally as wide. If you take a look at the commercial phase change systems like vapochill you'll see that they usually fit them into a compartment that is attached to the bottom of a normal case
You definitely have something doable started here - just a few other quick answers:
1- yes capillary is going to be the way to go, it's difficult to find an expansion valve that is small enough for our size systems
2- I hear stainless steel is incredibly hard to work with, just go with windshield washer fluid as your coolant and use cheap copper tubing available at any home depot
3- 12vDC compressors are available
4- quiet is possible - think of the noise a dorm fridge makes when it turns on, it's barely audible
5- car camper compressor - depends on the size of it, might be good - probably low power draw
6- what's a PC dehumidifier?
7- you are not stupid or insane, it's a fun hobby
*these are just my opinions - and I'm just a joe schmoe interested in the hobby like you - good luck and be careful!*