i think it is very nice that your MIL wants to plan for you, but knowing how my mom feels about me having her first grand baby, my mom would be let down if she didn't get to plan shower, i would let mil know mom was planning but might like some help and if mom is okay w idea, give mil her number to offer help
I wouldn't want to have a baby shower outside or have to rent out something. Your MIL and Mom could work together (MIL provides house, maybe some of the food, while your mom takes care of the rest).