Wide Angle lens for full format.....recommendation?

Since you don't mind manual focus why not look for a "broken" Nikon 17-35mm f2.8.   I have seen several threads about focus motors burning out on this lens.  I would assume that you could get the lens fairly cheaply.

Secondly, for night star photos, I would think that coma/astigmatism would be more important than absolute sharpness.  But take my thoughts with a large grain of salt since I have never tried it.

http://www.lenstip.com/239.7-Lens_review-Samyang_14_mm_f_2.8_ED_AS_IF_UMC_Coma_and_astigmatism.html