To answer your questions in my very own solid opinion:
1. The Lichterfelde* has enough room within their courtyard to utilize as a helipad. I've tested it out on the map and successfully landed the chopper right beside that large area by the gate. The MOI and Luftwaffe should have an ATC frequency controlled by the neutral Aviation Ministry as a mediator between aerospace feuds and regulation between the two. The other option is both factions sharing the airfield with each other.
2. Seeing as people love to make up random companies and units a lot within the game itself, the MOI's SL (being the military faction they are) can have a limited company/unit of men dedicated to aviation maintenance/flying in general. Although I think having civilian contractors is a great idea, I think it'd be strainful to get it up and going seeing as getting a contract done on the server is hard enough.
3. Thankfully, there's an oil company in town! (You know who ;)) Even then without an oil company, the MOI would have to work with the Ministry of Aviation as its own neutral state organization to utilize the airfield for fuel/maintenance and parts. What I believe is in order is an agreement between the MOA, MOI and the MOW to use the airfield jointly. (I know. Joint Airfield? I have to share with them? Blekh! Suck it up buttercup, you get cool helicopters so get along)
But these are my answers to your questions. If you have anymore, please don't hesitate to reach out to me either here or my discord: nightcire. Thank you