I am no expert at the game, and I am sure some of the guys that play German a lot can add more, or may tell you the best approach to defending in the first winter.
1. If you can take Moscow and Leningrad in 41, I would do it. Just know it may be hard to hold on to Moscow when the Russians start their counter attack. On the plus side, you capture manpower centers, factories and have more space to trade for time when the attack finally happens.
2. Forts are essential to defending. I would start preparing your defense several turns before blizzard starts, if you can, that way you have a good line without bulges, and can be entrenched.
3. You want to think out your rail lines ahead of time. Try and insure your rails follow the direction you plan on attacking. The better the supply situation, the better the units will perform.
4. Yes, especially mobile units behind the front can add in on defense and potentially save units in a combat that would otherwise loose. You want to protect your mobile units, try and have them in a city to limit the blizzard effects. If its close enough to the action, they can use reserve mode to help the front line units.
5. I don't believe this is a good use of build up, if the unit is already in short supply, a build up wont be effective. But hopefully someone else can correct me if I amn wrong here.
I think you have to be ready to give ground. Just stubbornly retreat as little as possible, and insure you don't allow units to get isolated. When the campaign season starts in 42, you will retake the lost ground, and hopefully win the game.