Hi Dave, Happy New Year.
Unless you are planning on selling the insert, I'd keep it and use it durnig those occasional -0* nights we get . The stoker will bie a lot more of a fill it and forget it type of heat.
I think you can do as you described : hook up the heat outlet from the stoker to the cold air return to your exsisting furnace. But if you are going to try to use only coal, you will have to have a separate fan only switch for your furnace, otherwise the other fuel soruce will burn as well to make up any shortage of heat demanded by your thermostat. The only problem I see with this is you may be corculating cooler than normal 'hot' air and this may not be as comfortable as the hot rush of heated air from a normal hot air furnace system.
I had to do a similar modification to my Propane boiler to keep it from burning propane when I'm using the room/zone thermostats to run the water circulating pumps. I put a separate thermostat on the propane valve itself, and us it as a low temp limit on the house, I run the circulating pumps all the time, so the heat baseboard units are warm and extract all the heat I can from the hot water, it is not as hot as the propane could get it, so continously at 120* is about the same as on/off at 180* [water temp] I'm still keeping the house at 65* plus, much warmer than the 58-60* with propane alone.
If the wood heated water can't keep up with heat demand, then the low limit thermostat tells the propane burner to make heat. So far it works pretty well.
Anyway back to your system. You state that the kitchen is on the other side of the house from where your stairway for convection is located, are you concerned that the kitchen may not keep warm enough?? Can you circulate hot air under the floor of the Kitchen??
If you spend a lot of time in the Kitchen and can get under the kitchen floor, you could install a water coil in the stoker and install under floor radiant pipes and heat the kitchen floor. Of course the coil could/should be used to heat domestic hot water too. Domestic hot water is supposedly about 25% of our yearly heat use bill. But probably only 5-10% during the winter months.
Just some ideas for your proposed system that came to mind