It would be tight, no doubt. The motherboard-GPU distance makes it pretty much 1U as far as pump/block height (or CPU cooler, for that matter) goes. So tubing would have to go where it can fit.
I don't see airflow for rear-mounted M.2 drives to be a problem. There's still a gap between the 2.5" drives and the motherboard, and the top fan overlaps the board a little so it can pull air up through it. Indeed, moving the drives to the space below the PSU and moving the motherboard over to the side panel to close the gap might result in worse cooling for back-mounted M.2, since the fan won't be pulling up as much air.
Still, there is some value in eliminating the rear-mounted drives and moving the motherboard over for more room for the CPU block, tubing, RAM, etc. The downside of moving the drives below the PSU is they would take up space that could be used for a second AIO with a short GPU (or a full custom loop, for that matter).
Yeah, motherboard and GPU extend all the way to the floor.
Right, dual 120mm rads is possible using a short card. Though as mentioned, moving the drives to the space below the GPU would get in the way of that.
