I think it's less heat generated by running on higher voltage which would end up running at less amps. You can run at 6S but your amps are higher which result in more heat.
Pretty much the same as you mentioned with ES ebike guys. Dual motor seems ideal if you are running your setup hard. I think 63mm would be better versus 50mm setup. 50mm should mainly be used for flat ground and/or you need to up the torque to put less stress on the motors. 63mm are definitely a better feel to it than 50mm. However, it also adds 1-2 lbs to your setup. If you aren't climbing any steep hills then it probably wouldn't matter much.
I really don't think 3mm HTD would fix any of your issues. The depth of the 3mm is just way too small IMO could be wrong. Let me us know if it works out. I'm definitely interested. I think GT2 5mm would be more ideal with 15mm width belts. You also could fix everything by going chains. The keyway also definitely helps. I would add it to your kits. I could only imagine the stress we put on set screws going up steep hills or riding the throttle hard. The pulley also gets mangled and tore up on the motor shaft and is hard to remove a few times. I assume this is due to the torque and a keyway would definitely help with it.
Chains are the easy way of fixing it and/or 15mm width belts. Chains are also an easier way of reaching a better gearing ratio. 10T Pulley doesn't seem to hold much while a 10T chain is probably more then sufficient. They do have some crazy belt setups on motorcycles. Haven't read what they use yet.
Batteries - As much as everyone would like to have 20ah on a board. That's a ton of weight. Not sure, how much weight you calculated but you'll end up easily being a 20+ lb board or so. 48ah 20ah might be around 8-10lbs?
In the end - the question seems more broad than anything. It's kind of tough to have 1 specific setup for an eboard to be lightweight 10-12 lbs, powerful 30+ mph, hill climber (20% steep hills), distance runner (12-15miles). It's more along the lines -- what do you need? Why do you need? Do you have the money?
Supposedly, highest output for power that we need while keeping the heat to a minimum?
@Vedder - Still want to hear what you have to say