Yes, no other accessories needed. The filter can be directly screwed on your camera lens. 

No, the ND filter from the magnetic set is an ND1000 (extends the shutter speed by a factor of 1000). But it would be technically difficult to construct a magnetic, yet adjustable, ND filter. I hope that K&F will bring more filters for the system onto the market over time. 

EOS-FX,You can use Canon EOS EF / EFS Lens on a Fujifilm FX Mount Camera body,, Your canon ef 75-300 1:4-5.6 is canon EF mount lens and fuji x-a3 is X mount camera body. so the adapter ring fit for your camera and lens. but pls notice, your lens is telephoto lens,we do suggest you could use one telephoto lens bracket that could be attached to the tripod to increase its stability. because the canon lens could not adjust aperture,so suing our adapter ring could not adjust aperture 

There does'nt appear to be a facility for holding a circular polariser on this system (i.e. the type you can rotate in the mount) 

i very much doubt it... it's worth risking your sensor and or your eyes...