Never used one but I have read a lot about them.
1) I really doubt that you could use a 0.79x reducer when using the hyperstar, I doubt it has enough back focus and the aberrations would drive you crazy.
2) Whether guiding is required all depends on your exposure time which will probably be quite short if you're using a CMOS. Whether you need to guide in narrowband really depends on the PE of your mount.
3) Yes! But there are ways around that. You have to bundle them into a single circular pattern across the aperture and it encompasses about 1/5th of the diameter. Google it and you'll find plenty of examples with C11/C14's.
4) Difficult one, probably but it is likely below the resolution limit of your image scale so you likely won't notice it. As for changing filters, Baader make filter changes and I think Starizona have them dedicated for the hyperstars. Even with the shorter exposures you'll still want a TEC cooled camera.
5) Light pollution is light pollution no matter the speed of the scope
