These are not allocated within FSX as I removed them. EZCA does not use Num2.
They are used by FSX, that's likely why you can't reassign them, when the "NumLock" is ON, the Num 2 is used to control the viewpoint to point backwards, and when the "NumLock" is OFF, is used to control Elevator Up.
I know that GSX uses Num2 when in Slew Mode for customizing parking positions
Yes and not only that. GSX DOES NOT use "Num2", at all. GSX doesn't have any notion of the actual key press, is not so dumb to respond to key presses directly, it only responds to FSX *events*, so it will still work and allowing you to redefine them in FSX. This means, the key used when inside the customize parking features, will be the one that are currently assigned to the Slew keys so yes, it happens to be Num2 by default, but that can changed and GSX will still see it.