Gents, Mazda sent a Senior Tech directly from Japan who supposedly specializes in the DISI to diagnose my problem. They tore into it, and came out with the following:
Bad ECT (coolant temp sensor)
Apparently on this vehicle, they can fail drastically without passing a bad ECT code.
P0300 hasn't returned in over 2000 miles, which is a good sign, but I got burned so bad on this deal (8 dealership visits, over 1.5 total months of service in 12 months of ownership), that I am always thinking it just might return.
Car runs smoother, feels stronger. I'm crossing my fingers hoping they actually got it this time.
For all of you that are frusterated with a P0300 that no one at your dealership can figure out, you might want to have them look at this.