So sorry you have to deal with this Tempest.
I would call an elder in her Hall.
The carts are for public witnessing. They are checked out by publishers (kind of like a library book)
Tell the elder it is against your wishes on your private property.
Tell him it is against zoning rules (if applicable)
Tell him this is a courtesy call and you are calling the Org in one hour (or whatever time limit you decide is fair to you) if the cart is not removed.
Tell him to explain to your mother that this is inappropriate use of the cart.
She is HIS responsibility scripturally.
The cart and where it goes is also his responsibility in the eyes of the ORg.