Hi, yes, the Pi Zero can be removed from the Node Zero's USB adapter board. The two microB connector sets between the Pi Zero and the USB board create a very tight connection (each connector set is fairly tight to begin with). I use a wide flat-blade screwdriver with the tip inserted between the two boards right next to the microB connector set closest to the side of the HDD. The screwdriver shaft is oriented perpendicular to the Pi Zero. One edge of the screwdriver blade is pushing on the edge of the PiZero board right next to the female microB (by turning the screwdriver clockwise) and the other edge ends up pushing on the face of the USB adapter board. Be careful that the screwdriver doesn't hit any components on the USB adapter board (there's a clear spot in that area). Then you need to do a "rocking" motion where you create a little movement/progress with the screwdriver, then pull the opposite end of the Pi Zero (with your left thumb, for example) to make a little movement/progress on the other microB connector set. It's a little tricky holding everything while doing this, but not too bad. Then go back and forth prying each side and gradually getting enough back and forth rotation of the Pi Zero to disengage both connections. It ain't pretty, but I've done it many, many times.
If you have trouble, or if the above procedure sounds too difficult, I can send you some parts so you can build a separate Node Zero "W".
My email is email@example.com