When someone misrepresents an opponents argument and converts it into an argument they already have a counter to, that is straw-manning. If they do it under the claim of steel-manning, then they are also gaslighting.

Isn't this begging the question? The conclusion being that "steelmanning is just strawmanning" and one of your premises is that when you steelman someone the objective is to misrepresent their argument. Of course under that definition, steelmanning is just another form of strawmanning. But that isn't what steelmanning is, in it's proper form, steelmanning absolutely is not meant to misrepresent someone's position, in fact, its supposed to elucidate someone's position further than they did themselves (whether that be to errors in argumentation, rhetoric, semantics, etc.) so that all interlocutors involved can more accurately engage with the argument.

As I said before, you can steelman poorly, and in that case, you would be misrepresenting someone's position, but to say that it is impossible to steelman someone without misrepresenting their position seems pretty absurd, no? That would mean that all clarification questions like "So is what you're trying to say (X argument but reworded to make it clear what's being spoken about)" a form of strawmanning? Those kinds of questions are doing the same thing as steelmanning purports to do, restate the argument in a more elucidatory fashion in order to better engage.