The EPA fine issue is going to be there even if you just install a catless x pipe from whatever company, but in reality the pursuit of such things is not a high priority.
Regarding the warranty, sure they can try to deny your claim, and they have the burden of showing that the part caused the failure. Although that sounds nice in theory, finding a decent lawyer to take a case with a $10-20k matter in controversy (1/3 of that of course if contingency) and going through hiring experts etc. will not be easy.
My warranty is gone anyway, but I ran aftermarket parts while under warranty with the expectation that BMW would put up a massive shit storm if I tried to get something expensive covered that they could even have a straight faced argument that it was caused by my parts.
Backpressure is necessary in some instances. I ran a E36 racecar with headers only temporarily, because I had just done an engine swap, and the shops I talked to about it strongly rec'd against running it hard like that. It can allow the engine to rev up too quickly and damage valves etc.