Even though TDI's are one of the most thermally efficient engines produced, the system should produce enough heat for complete heating. I never had problems with enough heat in my 98 TDI Jetta and believe me, i demand heat. I would say that even in the coldest weather here, -20,-30 oC i would enough after my regular morning drive of 10 minutes in town and then once i was on the expressway for 2 minutes i had HOT air from it.Even on cold days on the highway after a while i would turn down the temp a little. There is no real reason you wouldn't have enough if everything is working properly. Then again a short drive on a super cold Winnipeg morning would test any car or truck. Just keep the fan off or on low until it gets warm. Getting a coolant heater will help engine life on top of giving you heat a little sooner.
I don't think reducing airflow to your radiator in cold weather as "a bandaid and not a solution". If you look at many of the over the road trucks/18 wheelers they use the same concept when they snap on the vinyl air blocks in front of their grilles. The really fancy ones have multiple zippers so you can tailor the amount of airflow you allow so as to keep the engine at operating temperature and also allow proper coolant flow. If you rely on the thermostat to control the coolant temperature in sub-zero conditions you will end up throttling the coolant flow such that you can end up with hot spots in the engine which can lead to all sorts of nasty things happening.
In the Jetta that doesn't have any heat, can you hook up a VAG-COM while at operating temp and read what both temp senders are reading? You could also do that to the Jetta that does have heat and compare the readings to try and help point in the right direction.Could the thermostat in the Jetta that doesn't have heat be stuck open? What does the temp gauge read after a good run on the highway?