UPDATE on M3
As I cannot compel the dealer to purchase the car at a fair value for me to replace it (don't forget taxes, mods, time to find/freight one over, 3rd party inspections, etc), my only option is to have it fixed at the BEST possible shop I can find, have them pay for it, plus get me into a rental/loaner car. THIS they have done already. The car should be fixed within a week.
NOW, onto the diminished value: I have found the most experienced and knowledgable person in my area for this and he will prepare a report as soon as the work is complete. Based on the cost of the repair alone (approx $16K), his preliminary assessment is around $10K- $13K. The dealer has already suggested he will not entertain any conversation on DV, but that is what attorneys are for.
I, as the consumer, have a duty to "mitigate my loss". IOW, I cannot sit around for six months in a rental waiting for the perfect offer to come along. The dealer did offer to buy my car (for less than I owe on it) OR to sell me an '04 M3 with approx 15K less miles on it. It was keyed on 2 body panels, had curb rash all around, a smoker had owned it and it was covered in ash, plus the nose had been curbed (PLUS they wanted MORE money on a sale...I wouldn't have traded them even!). Not to mention no SMG, PDC, or alum trim, or cool mods.
The process is moving slowly, but we're at least moving. I'll let you know more when I do.