2. what would you sub for sherry vinegar? red wine vinegar?
I have sherry vinegar and I'd say that red wine vinegar is a much better substitute than balsamico with its pretty distinctive and unique flavour profile. Both the sherry and red wine vinegar have strongly flavoured, already fermented bases so that makes them kind of similar.
Maaaybe you could add a few drops of balsamico to the red wine vinegar for added depth but I'd say it is just fine without.
Mild vinegars like apple cider are not assertive enough, I think.