Apologies. It must be frustrating. It looks like you’ve gone through all the different attempts I would have suggested. Aside from the Log I’m not aware of any other method of getting diagnostic info. I’ll check up on it.
EDIT - After changing the folder path to no longer include spaces(ie from “C:\Unity 5.5.0a1” → “C:\Unity5.5.0a1”), it works!
Old post
A friend of mine has the same problem, he cut the folder to C:\Unity, then it works, then move it back to Program Files and it works