I’m not sure about that, it doesn’t make sense that valve would have two different board designs for the steam deck & a quick Google search shows many videos walking through the upgrade
This is incorrect, I got confused on this too. It is the exact same slot, the drive in the slot is just eMMC. Source - I bought the base version and slotted in a 2TB NVMe.
Nope the emmc is on a card in the m.2 slot. You just remove the 64gb emmc and slot in the 1tb nvme drive. I did it to mine when I got it. I kept the 64gb drive in a portable nvme housing.