I feel a bit like that mahts one were they keep on repeating PEDMAS time and time again.
SSA, shoes are not a big issue, paper plates are not a big issue, and the average person does not resent her mother in law buying her children shoes.
HOWEVER, every person seriously resents being controlled by a mother in law. A mother in law who tells a grandchild shabbos shoes are a waste of money, or inversely, your mother neglects you, i will buy you shabbos shoes, or even i don’t care what you do in e”y, but you are not coming to my chasuna in your dirty clompy sneakers, is seriously overstepping her boundaries, and is sure to be resented by her dil.
The thread of this post was not is it normal to buy your grandchildren a second pair of shoes for shbbos, i am 100% sure that in MOST cases both the parents and children appreciate it, the point was how hsould mrs. katz deal with a controlling personality.