How to handle an obsession about being 100% academically honest
Should I just accept that it's pretty much impossible for anyone to finish a degree with a 100% ethical standing?


No, I find that to be a rather unreasonable formulation of your predicament as well as a loaded question. What you should correct is not your notion that ethical behavior is achievable, but rather your unreasonable idea of how to achieve it, and your misguided view that there is some ideal level of "100% ethical standing" that is only attainable through extreme efforts of constant vigilance and constantly pestering your professors and TAs with annoying (and unnecessary, as you yourself seem to understand quite well) questions. In general in human affairs, trying to attain 100% of anything is an example of what is known as a category error -- the act of attributing a concept that is applicable to one realm of thought to another realm where it is no longer applicable. "100% ethical standing" is simply a meaningless concept.

I could go on and give a detailed answer touching on all kinds of philosophical issues and giving you a crash course on academic ethics, but a more lightweight approach seems like it will be more useful here. Basically, you just need to chill out. Relax, trust your instincts and common sense and stop worrying about this and you'll be fine, just like the vast majority of students who just want to complete their studies honestly and then go on to make use of the knowledge they acquired.