It has nothing do to with a perfect world. You can't base your case on speculation. Not only that but a dealer has no right to question you on parts that you put on your car. It's like a dealer asking you why you used aftermarket belts/hoses instead of OEM Mazda parts. Your also missing a very important fact. The onus is on Mazda not a dealer to prove that a part caused the failure (or abuse). It's like a dealer stating that you obviously don't care to spend money on service because you used cheap aftermarket belts/hoses instead of OEM Mazda parts. That is speculation not fact.
you can fight with them till you are blue in the face at a dealership, believe me I've been there. Your options are to battle this in court, or take it on the chin. This is why Mazda speed parts are so much more expensive as they are mazda branded and dealer installed so you don't have warranty drama.
You have to understand that dealerships do not want to do any warranty work at all. Warranty pays like the government pays doctors. They say what something is worth and how many hours it takes, regardless of what the real time turn around is. Any reason to have the customer shelling out for service at "trueish" time is money in the dealers pocket. If you really think that mazda is going to side with you the consumer vs a dealership when it comes to them spending money to service your engine... you need to look at the reality of the situation. Hell getting warranty to service EPA mandated stuff is a logical nightmare.
Fact vs speculation is how things are handled in court room. this is the service bay of your local dealership and you are already in a bind with the engine fubar, not a court room.
that same dealership also has the option to refuse service all together. I have been black listed from one local dealership for "embarrassing" their service manager and being rude. Luckily the guys 45mins from my house (not 5) know my car and myself and don't try to pull nonsense. They also had to fight with Mazda cooperate to "unblack list" my warranty way back when. It was more of a favor for them anyways, as they still call me every now and again when they can't figure something out.