Health, Are you assuming the minhag hamokem is based what the goyim (Arabs or Christians) that we live amongst do? Because that would be a mistake. It is based upon ones Jewish Kehilla. So if the Kehilla’s minhag hamokem was to have more than one wife, when they move from Morocco or Iran to Israel or the U.S., their minhag hamokem stays the same it always was, even if their new countries goyim do differently.
Also, what makes you opine that if she burns his food isn’t the halacha we pasken on (not considering the cherem, if we assume not everyone was mekabel the cherem)?