Lastly, I do not think it will feature any Wii U or 3DS backwards compatibility.
I'm thinking the NX handheld will support 3DS retro-compatibility. The NX handheld may be single screen, but even so it could display both 3DS screens on a single screen using scaling for instance.
I think the NX console will be retro-compatible with Wii U by using an optical media adapter that will cost extra (not included in base package). The GamePad in turn will be compatible simply using its wi-fi protocol. If the optical media adapter is meant for Wii U only, it might include the required Wii U chipsets for compatibility. If for instance the internal architecture of the NX doesn't support it natively.
I think that by default the NX handheld and consoles will be skewed towards digital only. The NX handheld may support 3DS through eShop purchases. The NX console will likely support physical media optionally, through perhaps that same adapter which can play Wii U games. Which means if you want to buy NX console games physically, you still can. But I'm guessing that practice will be discouraged, and over the lifespan of the NX console it's likely we'll see less and less games released in physical format for it.
Speculation is fun.