Ugh. You poor fella. I hate you had to go through all of that. New expansion valves are something I don't consider anymore for reasons you already discovered, but also because new ones are calibrated for R-134A.....which puts a crimp in how you, myself, and many others choose to charge it. When R-12 is used with an expansion valve calibrated for R-134A it won't work well at all. The old/original expansion valves are almost bomb proof. Unless they're physically damaged, they'll likely outlast the car. They can be taken apart and cleaned quite well. All you have to take note of is who many turns the brass allen screw is screwed in. Sometimes the end where this brass allen screw is will be deformed because of the flare fitting, but it can be manipulated out with patience.
When I restored Father's 450sel, I also bought a new evaporator. I can't remember which brand I bought, but I didn't have any trouble with the fittings like you did. It's been a little over two years, and none of the R-12 has leaked out. It still blows ice cold....However, the damned compressor (General Motors A-6) has started to make a hell of a noise. I'm quite agitated because I bought a brand new one. So I re-sealed the old one and will put it on. I spent a lot of money, and a TON of time getting the automatic climate control back to new and working properly, and I can't have Dad driving around with no A/C. Though, if I had to do it over again, I would have put the manual climate control in like you did. I just didn't have any of that equipment at the time, and I also wanted Dad to experience the full affect of it all.....automatic climate control included. haha Right now Dad is still getting over the expense of replacing the warm up regulator and the fuel distributor.....but it runs so well now. Both of us couldn't be happier.
The Sanden compressors are a wonderful compressor. They're very efficient, easy to adapt to most engines, and parts for them are easy to find. They have to be re-sealed every +/- 10 years, but they're worth it. The GM A-6 compressor on Dad's car is big, heavy, sort of unsightly, but it does a really good job. I thought about putting a Sanden on my W108 250se, but I had a brand new York lying around so I'll end up putting that on for now. It'll be using R-12, of course. haha
So yes, everything you did was definitely worth the time and effort. You now have a wonderful A/c system that doesn't leak! When they're done right they'll last a very long time. A few weeks ago I parted out a 280sel 4.5. It was a low mileage car.....around 80,000 miles....but it spent its life in Connecticut, so you can imagine what the underside of it looked like. Even the frame rails were rusted through, and the doors would no longer shut well. Anyway, it was a one owner car, and it came with paperwork going all the way back to 1972. I noticed it had the original Frigidaire A-6 compressor on it, and it had never been converted to R-134A. Well, would you believe that it still had over half of its required 2.2 pounds of R-12 in it?! And to add to the mystery of it all, the car hadn't been run or driven in 10-15 years. Likewise, but a little less impressive, my W109 had to have its R-12 topped off this past summer. It took 7 years of daily driving and regular seasonal used of the A/C to leak down far enough to be noticeable. I'm still happy. If I have to top it off every 7 years then so be it.