ATF went after Contender firearms over this very issue in 1992. Atf lost their case at the US supreme court.
https://supreme.justia.com/cases/federal/us/504/505/
I have repaired a cracked receiver by welding. It was not worth the time/trouble.
I don't think steel cased caused the crack. Ingram loves steel cased.
I cut 1/8" off the sides. Then remove only enough, at a bit of an angle, from the back to allow the catch to lock. I think this can vary slightly from mag to mag.
This is a closeup of the modifications of an original PS mag.
Yes, the original 2 stage Ingram suppressor endcaps had wipes.
The sound reduction numbers are not that reliable. There was no industry standard procedure for sound reduction testing.
The buffer might look fine but that's not a good indicator. Check the buffer by flexing it. Pull the edge back with your finger. Look for hairline cracks or tears.
Yes, that looks like an original buffer. The original buffers were black in color and were installed over both the recoil and ejector rods. That buffer is most likely rotten. Replace the buffer with a modern one.