From: Vermont, USA
I am in the same boat as the op. I really like the idea of this game and I love the manual. But if I play the game (WitE 1), I often can't figure out why something doesn't work. I've never quite figured out what kind of difference special support units provide (eg.how much cv? - when should I use a Nebelwerfer and when artillery). Any help to get into this game is appreciated.
I'm sure a lot of folks would be happy to help you, including showing you the ropes once the game is released and answering any questions you may have. There's a lot to learn historically but WITE2 is a great way to learn that history, it's sort of a game and an encyclopedia on the Eastern Front of WW2 combined into one.
Responding to your specific questions, support units are quite varied and some will provide a significant boost to CV (which if you manually attach them to a unit you'll see reflected in a change in that unit's CV value) while others will help more in just providing some anti-air, anti-armor or engineering abilities that a unit might not organically have. You can keep your support units at the Corps level (or the Army level, best for artillery) and let your digital commanders figure out when and where it's best to commit them, or you can assign them yourself for the most control at the cost of flexibility.
Nebelwerfers and Artillery are typically corp/army level support assets that are left there and committed to multiple battles, either on offense or defense, to add disruption and cause some casualties to the enemy's attack or defense. In large numbers, they can have a significant effect but one of their best qualities is the likelihood that they'll be able to support multiple battles in a turn. Historically Nebelwerfers (rocket artillery - the German equivalent of the Russian Katyusha) were shorter range, more limited ammunition and less accurate but rapid fire and when concentrated were terrifying to be on the receiving end of and could cause a lot of damage especially when you were facing a massed enemy force. Conventional artillery was longer-range, slower firing but more consistent fire, the workhorse of indirect fire support along with the very short range mortars that were organic to the combat units themselves.
In the game, reading through the manual section that covers artillery and support units will definitely help and looking at the in-game stats for the different unit equipment can also inform, but in general at this scale you can largely treat all non-siege/super-heavy artillery as fairly interchangeable and just make sure each corps and army has enough to help out when its needed.