Success, as far as I can tell so far. It is warmer today than the previous two days, air temp in the low 20s, but regardless of that I think the issue is resolved.
This morning, I got up and put my industrial heat gun about 18" away from the fuel filter assembly. Turned the air speed switch to high and set the temp for about 50% of max. At that 18", it felt comfortably warm but no where near hot. I let the heat gun run for 2 hours and checked it periodically to make sure things weren't getting too hot. The filter assembly eventually warmed to about 90F.
Once the filter was warmed, I siphoned the existing diesel from the tank. I didn't empty it but it was so low the fuel gauge didn't indicate anything.

I added a healthy dose of 911 and also an ounce or two from the white bottle. I then poured 4 gallons of winter blend in that also had been treated with the white bottle. It started nicely since I had the block heater plugged in for several hours. After coming up to normal temp on the gauge, I increased the RPMs to about 2K and let it run for 5 minutes.
Time to see if things were working correctly. I left the garage and started plowing snow around my yard. After 30 minutes, I parked the tractor never having a single issue of power loss. It was warmer today but still cold enough for the summer blend to start gelling, in my opinion. I have less than a half tank showing on the gauge right now so I'll hit town and get another 5 gallons of #1 at the Cenex station. They sell a lot of diesel so not much of a chance of having old fuel in their tanks.
The overnight temps are slated for sub zero during the next couple of days as things cool back down again so that will be the real test. Hopefully it will run as good then as it did today.
I want to take a moment to thank everyone that commented on my problem. It's great to have like minded folks here on the forum willing to share their experiences and provide input. It was very much appreciated. Thank you