FMJ ammo is not required. The gun doesn't know if the bullet tip is hollow or pointed. If I'm shooting 8mm Mauser, I use reloads since it's cheaper than factory, and I don't trust surplus. The initial quality of manufacture is usually a known thing, but how long and how well it's been stored is a big question. Sealed in a can doesn't mean that the powder hasn't started eating through the steel shells from the inside out. Blowing up a gun because someone is too cheap to use good quality ammo is inexcusable in my view.
The mysterious "sweet spot" isn't a mystery at all for a properly made gun. The guns that are glued together with bad welds, crooked and out of spec, are a lot of trouble. We've all seen some of the disasters that are out there. That applies to anything that's put together by someone who doesn't have the skills, whether it's a gun, motorcycle or car.
There is no proper length for a recoil spring. Spring length is only one of the many specifications. Resistance, diameter, compressed length, total weight needed to compress the spring, wire diameter - they are all required to be near the spec for it to work properly. Buying a modern spring (RTG) and using that is the simplest way to be sure of what you have. The recuperator springs need to function properly too. 70 year old springs aren't likely to work in anything the way they were designed.
What Wiselite does to get a gun to run is due to the problems inherent in the semi auto design. A semi 42 is on the edge for having enough energy to operate to start with, and with a less than perfect assembly, it's an uphill battle to make it run. I have no problem using the same booster for 7.62 and 8mm Mauser. I trade the top cover, feed tray, and barrel, and that's all. Mine runs fine, but I only use quality ammunition, whether that is military surplus 7.62, or reloaded 8mm Mauser or 7.62.
What the M53 was made with from the factory is not something I'd know. The M53 is a direct copy of the 42, so everything except the buttstock and buffer works for either gun. The standard bolt should not be replaced with a heavy or very heavy bolt, since nobody seems to have the heavy or very heavy buffer that it needs to run it properly. The bolt and buffer have to be matched, and the people who switch the bolt without matching it to the correct buffer just beat the gun to death, then blame the gun.
The German manual says the 8mm uses either 14 mm nozzle or the 11.5 mm (for a faster rate of fire). The 11 mm nozzle is only used with shooting 7.62 ammo. I don't read Yugoslav, and don't have any idea what they made in theirs or what their manual says. I've only ever seen heavy and very heavy bolts that were German or Austrian (MG74). The Yugo bolts I've seen were all the standard weight, the same as the German standard bolts.
The lack of knowledge about the MG42 is what caused me to start translating the original manuals 12 years ago. I asked 5 people how my gun worked, and got 5 different answers. That means at least 4 of them had to be wrong. As it turns out, once I explained how the gun worked to a few people, and that I got my information from translating one of the original manuals for the gun, I started selling a lot of manuals. Now that there are a few thousand MG42 manuals out there, and a lot of people who have built semi autos, how the gun works is no longer a mystery. It's certainly a different method of operation than the normal semi auto rifle, but it's easy to understand and make work, perhaps less so when it's a semi, but it's still doable.
Swapping parts is a horrible way to try to make anything work. It demonstrates a lack of knowledge about what is happening, and ignorance about how to fix it. Ignorance is curable through education. Most everything that can go wrong or be wrong with a 42 is discussed somewhere on this board. People just need to read, or at the least, ask questions. I don't require someone to buy a manual for me to give them the answer, they just need to ask. I don't know everything there is to know about a 42, but I've read 10 manuals in two languages on the gun, so I may have a head start on some people.