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error -- the act of attributing a concept that is applicable to one realm of thought to another realm where it is no longer applicable.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.
First of all, while ethics and rules should ideally be aligned, they are not the same. If you spent a reasonable effort to inform yourself about the rules and to apply common sense, accidentally breaking some bizarre rule is not an unethical action. This does not only help your conscience: Resorting to the ideal of an alignment of ethics and rules is a reasonable defence against non-obvious rules in situations where repercussions are decided on a per-case basis. While you may face legal consequences for importing Kinder Surprise to the US without knowing about this being illegal, you will almost certainly not be expelled from your studies for something like this.
I have yet to encounter a case where somebody claimed lack of knowledge as a defence against accusations of academic misconduct where I would not consider the lack of knowledge itself to be unethical, i.e., the accused acted unethically by neglecting to inform themselves. (Note that in most such cases, I do believe the excuse to begin with.) Going by your question, you are sufficiently informed about academic rules to satisfy my ethical standards. Now, all that is left to do for you is to use your common ethical sense and act accordingly.
All that being said, some rules of thumb for cheating and other academic misconduct are:
Adhere to the Golden Rule.
In no situation may you present somebody else’s work as your own (plagiarism). This in particular means that you may not let somebody else do your work.
In exams and similar, all resources (pocket calculators, books, notes, communication devices) and communication are disallowed per default.
Everywhere else, all resources are allowed per default.
If there is no practicable way to prove that you broke a rule, it likely doesn’t exist. (Note that letting somebody else do your work can be checked by having you explain your work.)
If something’s main purpose besides learning is to prepare you for an exam (e.g., exercises), it’s your own problem if you get too much help.
If you cannot think of any (good or bad) reason why something should be disallowed, it probably isn’t.
Regarding your individual examples:
A little while back I asked my professor if thesauruses were allowed, and if I had to cite them.
The work contained in a thesaurus is the collection of a list of synonyms. Using a thesaurus to find the right word to use in a text does not pass off that work as yours. (If your task was to compile a list of synonyms for a given word, you would indeed need to cite.)
My professor for another course mentioned that certain calculators were not allowed on tests (the kind of calculator I currently have). I wondered if I could still use my calculator on assignments […]
The reason why advanced calculators are forbidden in exams is that they can store information and completely automate certain tasks. If you use a calculator in assignments and you still can explain how you arrived at your result, that’s usually fine by the rules. It’s your fault if you do not sufficiently learn the respective technique (for the exam) or understand the underlying concepts (for your future). Also, a hypothetical ban of calculators could not be enforced.
I was wondering if I could use my course notes while doing an assignment.
Besides helping you maintain your attention, the entire point of taking course notes is to allow you to use this information when learning – which is exactly what assignments are for.
I want first of all to follow up on a comment made by @Nelson. You speak of an obsession and an obsessive need to behave ethically in all your student endeavors. Could this actually be an obsession, i.e., a mental disorder? If you (and/or your close friends and family members that are familiar with the situation) feel there is any possibility of this, I recommend that you look into it, e.g. starting with student health / counseling services. The point is, if you do have OCD or something similar, then your course of action will be quite different from what the other answers recommend.
If things like OCD can be ruled out, then it seems that your problem is one of a lack of confidence in your understanding of the rules. This need not be pathological: I agree that in order to act ethically according to the norms of some particular group, you have to have a good cultural understanding of those norms. Rather than asking about every single doubt as it comes, please consider getting some more systematic help in understanding the culture of academic honesty at your particular institution. If you are at a US institution, it is virtually certain that there are people on staff whose job it is to be experts in the local culture of academic honesty and inform the university members about it. Making an appointment with such a person could (perhaps) go a long way to setting your mind at ease.
I suggest that you ask in particular:
Can you list some things that I can be confident it is ethical to use when doing assignments unless specifically informed to the contrary: e.g. dictionaries, thesauruses, calculators?
Are there any contexts in which it would not be permissible to use my own course notes when doing an assignment? Is it the instructor's responsibility to specify them?
I will end with two comments:
First, as a math professor I find it completely reasonable to ask whether calculators are permitted when doing homework assignments. There are times when they won't be. Asking once per course is not pestering anybody.
Second, both of my parents were English professors and there was never a thesaurus in our home. My own unsolicited opinion is: choosing the right word is a critically important writing skill. If you have it, you don't need a thesaurus. If you don't have it, then -- because a thesaurus just lists approximate synonyms without analyzing gradations of meaning -- using a thesaurus could lead to replacing a better word by a worse one and could make your writing sound stilted and artificial.