You got me thinking...
Plot twist, Nana isn't just an unwitting enabler. She is a full on cuckquean. She acts all sweet and innocent, but no, she is manipulating the MC into building his harem because that, more than anything, gets her off. She runs the whole town like a mob boss, sweet little old Nana sitting there collecting dues from all the property she owns. That's how we'll get the "mayor," little more than a figure head in this town run by Nana. At the end of it all she'll reveal that this was the plan all along, build a harem for the MC out of all the lonely unsatisfied women in Nana's town... Grandma's House? No, Grandma's Town...
Or maybe I'm just going a little nuts... could go either way...