Need help getting this heat-moving experiment to work

Consider going to a smaller fan maybe 100cfm; it will run cooler due to lower static pressure. Also locate the fan at least 5' away from the stove and duct it to your original contact point. Pushing 300-400cfm through the 4" duct over any significant distance will put a decent load on the fan causing it to run hotter (don't know the length of your run).
As far as safety, can't comment without seeing entire setup; just clean dust out of the setup at least 2-3 times a season.

The problem is your axial fan...air over the motor. If you're going to run that, just get a squirrel cage blower with the motor outside the air stream. You can suck up all the hot air you want and discharge upstairs, but the motor is outside the airstream and will stay cool.

Never thought of that!