did you walk with the contractor with the quotation, sometimes, they "forgot" to insert certain part and stated that whatever not in the quotation is not cover. May not be they are cheating, but sometimes,people forgot.

If the contrator is not sub contract out, you properly get what you want, else, there may be mis communication.

Based on your quotation, they put there the grill is 1.5 inch (hollow), the 1.5 inch is not the issue, you need to ask specific the thickness of the grill, because the thickness of the steel is the impact if the grill can be easily cut.

You has nice yard, the yard is the easy target to come down, so the thickness of the grill play roll for your family safety. Maybe i think too much, but i saw your initial picture of how the ceiling got the hole, they may be some occupancy during the abandon period.

solid definitely better if budget is not a constraint.