Your husbands Rav (ergo your Rav) is correct. Halachos of kibbud av do not require that you follow all her suggestions. It only regulates how you respond to her, or not.
Regarding shabbos shoes, seriously, ask her why it matters what shoes your son wears? Is she paying for them? Perhaps it is her way of saying, we have to cut back on support? Does your mother in law wear her weekday shoes to a chasuna or lihavdil on shabbos or yom tov, why should your son?