A better approach is to use the less loaded "have you considered doing X here?".
If you find yourself needing to demand others not take offense at your questions, you might wish to try a different approach.
A "Why[...]" question asks for why.
Assuming that "Why didn't you just do X?" isn't really just a disingenuous way to say, "you're an idiot; do it this way instead", then you should ask for what you want and ask the why question, not the yes-or-no question. In the case that it is a disingenuous question, then that's something that's covered adequately by #5. Suggesting the avoidance of sarcasm is already broadly applicable enough to cover using questions when you really mean to make a statement. (Which is not specific to code reviews—it's as obnoxious in real life.)
After that I usually reply with something like "Could you add a comment and/or test highlighting this reason? (So the next time someone modifies this code they don't make the same mistake I did.)"
Definitely better results all around.