I am not 100% sure, but I would bet the wire feed for seat heaters from the factory is either under the drivers seat carpet or tucked up in your dash somewhere. The seat heater positive wire comes out of the main bundle of wires in the dashboard, goes down by the center console and then to the swithces or the heaters depending on what year. Since the wire is in the main bundle, and I doubt they would have used different harnesses for cars with and without seat heaters the wire is probably there. If I remember right it is a blue/yellow striped wire in 1984 and comes out of the main bundle around the steering column
I use the stock plastic/aluminum fuses. I am not disagreing with your calculations, I am just stating from experience, that the #12 fuse will run both of my heaters without blowing (and yes both heaters are working). Plus #12 is where the heaters from the factory were connected to. I am assuming some engineer at Volvo checked out the load on that circuit.
The heated seats on my 89 are on fuse #13 from the factory, and it is 16 amps. But that circuit also has other stuff on it.