Does a central A/C with copper lines that are too small cause compressor and diaphragm failure?
If the lines are two small it will change the efficiency and work the compressor harder. It is tough to fully clean the old mineral oil out that was used in older units. If the R410 smells acidic this could be an indication that the old oil was still in the system and mineral oil and ester oil don't mix. If there is old oil in the system this can cause early compressor failure. I am guessing what he called a diaphragm is the throttle valve or TXV that regulates the flow into the evaporator this could be damaged and part of the reason your system is not cooling. There are many electronic testers that can detect R410 at very small levels if a leak could be located a bubble solution can pinpoint it and there epoxies that may be able to slow or stop the leak but silver solder is the best way (not on a charged unit though) . The possible acid in the oil has caused damage already and compressors are not reparable. I would run the unit until it fails with a full charge it may last the season and shop around for a new system (get several quotes for the complete replacement). I am sure they did not want to put there name on a system that is damaged and have you upset with them as well as the first company for trying to save a few bucks that cost more in the long run.