I don't have much to offer you for the other questions, but one where I *would* make a suggestion is to forget the HRM. So-called "zones" are iffy and variable across individuals even with the idealized age-based formula, and god knows what it would be when factoring in type I diabetes. Guessing the magical point would be pretty arbitrary and even if you could find it, I doubt you could hold it steady for 26 miles. And, if you want one anyway, get a cheapie one and stop saving your money - I got mine at a cut-rate sports supply shop for the equivalent of about 20 bucks.
Other than that, everything I could say would be speculation... but I would suggest trawling the paleo-friendly physicians list on this site and seeing if you can find professional advice. I doubt you'll come up short for support. Meanwhile, let the speculation begin!