Being around my biofamily made me realize all these things that used to be normal for me that I now consider super shitty behavior. Using loopholes to escape things you agreed to do: this is treating someone as your enemy. If you know what the point of their request was and you are deliberately looking for a way to avoid that, you shouldn't have agreed to do it in the first place. The ethical thing to do would be to go to them and say, "I know I agreed to do this but I'm not comfortable with it, let's renegotiate." You should be able to trust that if someone ends up breaking an agreement, it wasn't due to lack of effort to honor the spirit of the agreement or renegotiate to one they could honor. You should not have to think of all the ways people could say yes and then blame their not-doing-it on your phrasing in order to lock people into an agreement they can't wiggle out of. They shouldn't agree to things if they are willing to try to wiggle out of those agreements.
I hadn't realized that it was M who was responsible for inversely-teaching me to not poke people in their sore spots or say things just to irritate. That is disrespectful. If you know something causes someone upset, be sensitive if you HAVE to discuss it and if you don't have to discuss it, just don't do it. Deliberately causing distress for one's own amusement is not making a joke, it is being unkind and disrespectful. Also, I realized how effective my method of stopping that behavior is: describing it and explaining how it is unethical. If you protest with distress, that gives them the 'fun' they were looking for. M's 'jokes' may be technically harmless (implying to P that they resemble someone P thinks is unattractive/unpleasant) but the effect is where you know the ethics of a thing.
(meta-note: I haven't had time to do more than write lately but I promise I'm not ignoring anyone, I just want to keep momentum with writing. I will respond to posts and comments when I next have time)