Request does not mean, “there is no reason to buy shabbos shoes”. That is an opinion. Kibbud Av does not require you to subscribe to or follow the opinion of your parents. Now, if your parent forbade you to buy them shabbos shoes, that is something else entirely and I would suggest going to your Rav for advice how to tell them to keep their noses out of your business.
“She thinks shabbos shoes are a waste of money, and told my teeenage son that even yoisef Moikir shasbbos didn’t have separate shoes for shabbos!”
He didn’t have a meddling shvigger either, just saying.
“Am i supposed to bend to her opinion even on such things?”
Well, are your in-laws supporting you? If they are, right or wrong they feel they can dictate how you spend their money. If they are not, then politely say, this is how I choose to spend my money. Does she wear her weekday shoes to shul on shabbos? Why should your son.
“She claims that disposable tableware is bal tashchis. Is she right?”
Is she offering to wash the silverware and dishes after each meal?
“I think that if it is cheaper than the price hired help would take to wash real than it is a good investament.”
Tell her so.