As for the leather belt/shoes analogy, surely that is like someone deciding to become a vegetarian and then finding a beef lasagne in the freezer a week later and eating it anyway, because else it would be a waste? I appreciate your point that using stuff you already have has no effect on demand (and hence, doesn’t encourage further manufacture of leather based goods), but would you feel the same about someone with a old fur coat in their wardrobe? Would it be acceptable to wear that?
NB not trying to ‘catch you out’ here, I think there is a big difference between a pair of leather shoes (which I would wear) and a fur coat (which I would never wear) in terms of ethical acceptability, but really they are effectively both animal skins? I guess it’s like you say, you have to do your best and find what you are comfortable with.