obrother, can you find a way past the customer service representative in order to escalate the matter within the Mitsubishi organization? Surely there is someone there who understands the pseudo-technical nonsense that is being spewed and will realize what the outcome will be if it's taken to court.
This is potentially a big-ticket item so you rightfully need to tread carefully. Before turning the car over to the dealer, here's the approach I would take -
1. Obtain written statement from the dealer explaining the rationale for the additional testing required and their assessment of what they believe the impact of this accessory (the diesel heater) has had on the battery pack, and why YOU have to pay for the testing. If they don't want to put it into writing, pull out a camera or smart phone and insist that they give you that reasoning on-camera.
2. Insist on a presentation and analysis from the dealer of all the possible results of this testing and why those results would not be covered by the battery warranty. It would be good to record and also have someone technically knowledgeable with you for this discussion.
3. If you are required to pay for this additional testing, obtain, in writing, a statement from them stating that they will refund your testing costs as part of the warranty coverage. I would be very reluctant to go this way because I challenge that in (2) above they can show that the cell failure was caused by anything external to the pack.
Before turning the car over to them I would be inclined to challenge them in small claims court over the cost of this additional battery testing.
Is there not a consumer protection agency in Canada? I would contact them for advice as to how you should proceed.
In the US we have the Magnuson-Moss Warranty Act. For example, extracting from the following link: https://www.consumerreports.org/cro/news/2014/06/can-an-aftermarket-part-void-my-car-warranty/index.htmThe Federal Trade Commission says, “The manufacturer or dealer must show that the aftermarket or recycled part caused the need for repairs before denying warranty coverage.”
Just out of curiosity, how much 12v current does that diesel heater draw?
All this is beginning to leave a bad taste in my mouth regarding Mitsubishi (only Mitsubishi Canada?); I still have to believe it's the result of technically un-knowledgeable individuals in the decision-making chain.