It sound like the starter motor itself then or the solenoid/bendix mounted on top. Maybe the plastic solenoid cylinder gets warped or something or contracts in the cold to prevent it from engaging. At any rate it would be replacing the whole unit or at least disassembly to see if a problem can be found. Just IMHO anyway.
Check and clean the ground connection on the engine block, possibility 1. Wind chill has no effect on a cold engine, only temperature. A block heater might be a necessity for this car, not a dipstick heater, not a magnetic go on the oil pan heater, a block heater that is put in in place of a freeze plug. $150 or so last time I checked. 15 degrees or below plug it in. I guess you should check to see if your original invoice had a cold weather package, you may have one and not know it. You could even go to 850 cca as a plan b.
When the key is twisted to the CRANK position is there any noise from the starter or a relay?
You said when you pour hot water on the starter it works 100% of the time. That tells me it’s the starter. Cold causes things to contract and grease to thicken. Either could be the internal problem in your starter. Replace it and move on.
Progressively warmer temperatures are resulting in failures to start - so whatever the problem is seems to be getting worse. However, a pot of hot water poured over the engine above the starter still has 100% success - even when it was below 0 degrees!
Thank you all.