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. 

Daa 1.8 FE has no OSS, therefore cheaper 

If the gh4 is a micro 4/3rds system then it will work. I have quite a few Nikon and Sigma lenses including the 50mm afs and they all fit and work perfectly. You will have to set your aperture and focus manually but the camera will make all the necessary exposure adjustments itself. Don't forget on a 4/3rds system your 50mm will be 100mm based on a full frame. Make sure your camera is a micro 4/3rds and not a 4/3rds, I have both. 

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...