sounds like the vendors here are recommending a good approach. Strong-arming the dealer to say you guys do it unless you can prove cause-and-affect is the wrong approach. They'll just tell you to go F* yourself.
You should work with the strategy of removing each aftermarket component in the troubleshooting process and work with the dealer on this. I'm sure they're happy to help, but you gotta meet them halfway.
with all aftermarket components, needless to say, you gotta pay to play. These are the risks involved. Some more riskier than others.