I agree with your point on the typo's, meaning altering should be counted as a mistake unless obviously accidental. If there's suddenly a comma in the middle of a word or a repeated letter, I generally don't count that as a mistake, unless it keeps happening in which case it's a sign of the translator not being careful.

The second point is a hard agree as well, at that point you start to wonder whether the right one was perhaps an accident haha!

As for the third point, generally I would hope that the context wouldn't change because of a better word. The way I imagine it is that for instance someone decides to use "compilations" but the LM thinks that "abstracts" might fit the context better. The meaning of the sentence shouldn't change, but one word might fit the tone of the project better than the other.

I agree with you on the last point as well! Everyone has bad days, but consistent failure to include something should indeed get a lower score. I like moving from "bad" to "unacceptable" instead of going to "unacceptable" immediately, I hope I won't have to use it but if so I feel like that's a fair step to take!

Thanks for your reply!