I still don't understand what happened with the insulin. But I gather it was a med error of some kind, you recognized it as a problem, took steps to protect your patient, you wrote yourself up on it, hopefully corrected the deficiency and heard nothing back about it again.
As far as the surgery prep without orders.... If I was told at report that surgery is in the AM, the patient tells me they are having surgery in the AM, and the note says there is going to be surgery in the AM, I would have gone ahead and done the prep too even if there weren't orders for surgery prep.
I think you made the right decision. It wouldn't have made sense not to do a surgery prep given the information you had. And it would have made you look like an idiot if you hadn't done the prep. Yeah, we need orders for stuff, but it's not like a night before prep is a big deal, unless we're talking about bowel prep or something. We do a CHG wipedown and NPO.
From all you're telling us it sounds like you got the shaft. It doesn't sound like you made any horrible do-not-pass-go mistakes. Better to move on to make some lemonade with these lemmons as others have suggested. Only thing is I would have probably gone the union route just on principle, but I would have declined to work under that manager again.
I'm not sure how to tactfully list this job experience on a resume, but I think you need to list it? Maybe others will chime in with suggestions? Good luck in your job search.