1. If stars are displaying comma in all four corners then yes, a field flattener will address that.
2. If you have comma in pne corner and not others then that might be an alignment issue with the camera focal plane relative to the line of sight. This will certainly cause focus error, as part of the focal plane is further away from the center and part is closer than the center to the objective.
3. Vignetting is pretty much always present which is why we do flats. Flats basically are used to correct the transfer function of light passing through the optical system as a function of radial distance from the center. If you have light somehow entering off axis and getting scattered around I wouldn't try to eliminate it by a flattener. Taking a series of flat images won't help either as these are done with a uniformly illuminated aperture.
Image looks nice. Great focus Tracking obviously good enough to render nice round stars. The star field is very dense around there, and after looking at other images I think what you might be thinking of a sky glow might well be faint nebulosity that appears in all other photos I inspected.
It seems clear for your 80mm, the PMC8 guiding is doing just grand. If you were not guiding, then Bully Bully. 240 secs is amazing.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired