I don't think the TRX has to have a "2500 frame" nor have i read anything to indicate that it will. This could just as easily (and much more likely) be a 5th gen ram 1500 frame slightly modified to fit a 2500 8 lug axle, than a 2500 frame modified to fit 1500 front axle, bumper, cab, and bed... not to mention this wheelbase (crew cab 5-7" bed) is not available in the 2500. The 5th gen 1500 frame should be close to if not already is strong enough. Or they could just increase the section modulus of some of the rear frame sections where needed (like F150 does)
But differences between 1500 and 2500 frames probably aren't huge in some places anyway. At least they werent for 3rd gens. 2500 was just 1" or so taller in the rear than a 1500. Made it pretty easy to mount a gooseneck hitch from a 2500 and put it in a 1500.
Regarding the front axle it could well have larger components over a standard 1500. We dont know yet. Just because it has 6 lugs in front doesnt mean it cant have larger half shafts, more splines, or beefier carrier assembly. I'd put money on 4.10 being the only option but who knows just guesses at this point. I'd also bet money that it wont have cheap plastic components that will break like the raptor...
I see your point on the fender flares vs new front and rear fenders. As stated this is a test mule. Production TRX may well still get new fenders (as well as hood, front bumper, or grille) but testing without them makes it so they don't have to cover the entire truck with all the black crap. The RamRunner kit had fiberglass fenders. Ramrunners also destroyed the Raptor of the day, and that was still just a standard Ram 1500 frame with a suspension kit. That said it wouldn't surprise me one bit if they didn't go with all new fenders as the widebody challenger and upcoming charger just have fender flares. I'm not complaining because we get 700-800 HP for under 100K. Oh and it wont be a turbo V6
TRX will be based on the 1500, not the 2500. The problem is trying to put all 707 horsepower down to the ground. Remember the Rebel TRX Concept was only 575 horsepower.