Okay, I'll try to be a bit specific on what I'm asserting:

1. For any scale of "ethicality" what matters is not only the actual scale, but also the "zero point". On a hypothetical scale of -10 to +10, there's a lot of choices that are not the most ethical choice providing the most utility or making no sins or whatever model of ethics is used, but are "above zero" - so they are ethical actions (ethically permitted actions) despite not being most ethical choices. Most things that we do fall in the range between, say, 0 and +5 on that arbitrary, hypothetical scale - they're ethically permitted but far from the "most ethical" possible acts. E.g. it's ethical to try and follow the effective altruism movement principles; but it's also definitely ethical to perfrom ordinary altruism.; it's ethical to abandon your life and go to a poor country to feed starving people, but it's also ethically permissible to not do that and simply live a good life. Otherwise we might as well say that everyone who's not devoting their life to charity is unethical, and that's not what the word means.

2. To adjust for moral relativism - there are many moral standards, however, even in moral relativism we (or at least I) expect them to be mostly aligned. E.g. what's +7 for you might be +10 or +5 for me, but it's very unlikely to be -7. If someone's personal ethics or religious persuasion allows and even mandates them to, for example, rape and kill babies, then we simply mark their relative morals as unacceptable (unaligned?) and invalid despite generally accepting some relativism. And relativism is tricky - we do accept some relativism - if someone asserts that not following their exact moral code to the letter (which most of the society doesn't do) automatically makes someone (e.g. most of the society) immoral and evil, then we consider their morals, or at least that part of their morals, as not aligned with widely accepted morals, extremist, not valid, and ignore it. I.e. the "privileges" of moral relativism seem to be granted only to those who also grant others the same privilege.

So I'm working with the expectation of not-absolutely-objective but still somewhat aligned moral principles, making an assumption that if something seems definitely permissible for me; i.e. not even close to 0, far from the (admittedly fuzzy) line of what's permitted and required in my opinion; then for others with different-but-still-reasonable ethical priorities it might be, at worst, mildly discouraged but not breaking any major taboos.

3. Calling someone unethical or immoral is a strong accusation. It is justified if and only if the action goes "below zero" on that scale; if the agent has broken some taboos or significant moral principles. It's not appropriate if the agent has merely acted unoptimally or "less ethically" as they could. It's appropriate if the agent has failed in some ethical duty, if some evil act was done, but it's not appropriate if the agent has failed in some ethical "opportunity", if they did not do something that is nice but not morally mandated.

4. Teaching someone how to use an useful proprietary tool is not unethical. Teaching someone useful skills is a good act that helps that student, does not impose any undue harm, does not violate any person's rights or moral imperative, it's strictly "above zero" on a moral scale even if it would have been more good or "more ethical" to do something else e.g. teach some free software instead. All the arguments made above for ethicalness of teaching free software instead of proprietary tools go into the category of "it would have been better to do that instead", there was no argument made that teaching proprietary tools is actually harmful or evil, and that there's some specific harm to society that outweighs the benefit to the student. So I did not see any actual justification why the act should be considered unethical (only assertion that doing something else might be better), but there was an assertion that anyone doing so is unethical.

5. Unjust accusations are harmful. It's offensive, harming people without appropriate reason or justification, and harming people does violate most moral principles. I assume that this is not what's being debated here - I would assume that the grandparent poster would agree that unjust accusations are harmful but would rather contest/debate the position is that these accusations are just; they might question my #4 assertion, but not this one - however, I might be mistaken, of course.

6. I'm not asserting that you should not call out unethical actors because that would offend them; there's nothing ethically wrong with just or justified accusations even if they turn out erroneous because of a honest mistake. But I am asserting that in any reasonable debate it's appropriate, polite and even an ethical imperative to give some benefit of doubt (not "innocent before proven guilty" beyond all doubt criteria, we're not proposing to execute or imprisin someone, but at least some reasonable benefit of doubt) before making any accusations. This requires some serious consideration whether that act is actually unethical (according to the criteria of #1) or it's merely less-than-optimal e.g. failing to signal some support to an ethical movement.

7. I am asserting that making unjust accusations of unethicality is itself unethical (i.e. not just suboptimal, but "below 0 on that scale", breaking moral imperatives). This is a much wider issue than this debate on free software, I've seen such accusations of unethicality very frequently misused (IMHO even intentionally) in recent political debates on both side, and I believe that this misuse of accusations is harmful behavior.

Going back to your particular example of the statement "Some people would call an action unethical despite the potential of offending the actor, and although thus speaking out is also a valid ethical choice, it's not the only ethical choice.", it is something that seems reasonable to me - it's permissible to call out unethical actors (though note the abovementioned difference between calling out actually unethical actors versus claiming that an actor is unethical without any grounds to do so), and it's permissible to not call out unethical actors; I don't see any contradiction there.