Length is actually NBD for me. I'm 5'-11". With my DS seat forward I have plenty of room. The transition is pretty good, but needs to be improved a bit as the platform likes to slip off of the rear drawers if I don't set it up just right. I'll work on that next year.I've slept in the back of mine also and as you mentioned, head room is one issue, the other is length. I have to have the passenger seat moved forward to be able to stretch out, but since I didn't have a platform, I had a gap between the front seatback and the end of the folded down middle seat which I had to back fill with "stuff" to bring up to the folded seat height for my head. I image you have way less head room with your platform but at least you don't have uneven transition between the rear seats and the folded down middle.
Didn't see the lower platform in there at first, but yeah, that would provide you with 8" plus more headroom than I have when sleeping on my drawers.When I'm out wheeling/camping alone I sleep in it all the time, but it's a little harder with two people. My normal platform is actually more like yours and is only the 60% side since it doesn't need to account for the fridge. But the fridge is tall and it needed to be flat for this time. We're backpackers so tenting isn't bad for us, but it's gonna be cold and we don't have a 4 season tent.
TBH, I've gotten every bit as cold sleeping in my GX (in the low teens) as I have in my backpacking tent. The rig does break the wind and gets you off the ground - but low static temps are static temps, and the rig cools down fast.
I finally broke down and got a NeoAir Xtherm air mattress after one very, cold, uncomfortable, and near-sleepless night in my rig. Amazing how much warmer it is than my normal NeoAir pad. You wouldn't think sleeping on carpeted drawers would suck the heat out of you like sleeping on the ground in a tent, but it definitely can.