What about humidity with older FG's?

It's plywood. Humidity is less of a concern. Many of them survived sitting in an attic or basement for decades with minimal damage. Large changes in humidity will affect it slightly, requiring retuning, but I've never seen a plywood Yamaha with a cracked top. It just isn't possible.

Extremes in heat and humidity can affect glue joints though. But that would be extremes outside of normal summer/winter changes. I've wanted to leave a vintage Yamaha in a car during the summer to see what will happen. But I cut them up instead to be able to see the inner bracing and neck joint.

I'm in Connecticut with oil heat also. Most of my guitars are in a dry basement that the furnace pulls air from and all the cold dry air comes down there when someone opens the front door. I run a large humidifier in the area (that is open to the upstairs), going thru 2-3 gallons per day and have a tough time keeping the humidity in the low 30's when it's really cold. All the solid wood guitars stay in their cases with humidity bags. The vintage Yamaha's stay out all winter. I've never had a problem with one of them.