Here is the question: are choppers and airplane then considered SOFT units? So an infantry unity could fire for example with a M60 but not with a Panzerfaust to an airplane?
I guess you could think of them as soft skinned, although the Haunebu III behemoth would take offense at that, in order to easily remember that they can be attacked by both AA and unlimited ranged HE weapons.
It only makes sense that you could attack a high speed flying craft with either a guided projectile or by raising a wall of bullets in front of it. AP weapons such as the Panzerfaust 3 aren't capable of doing either.
About the spotting: there are two units in a blocking hex and they benefit of the concealment bonus. One of the two opportunity fires and becomes finished and thus loses its bonus. Does this apply also to the other unit which did not support the fire in order to not be marked as finished? In other words, like in LnL for example, if a unity fire is the whole hex "spotted"?
This question is related to your earlier one about units not being able to block LOS. At ~150 meters from hexside to hexside, the area of a hex represents roughly 15000 m² (160000 ft²). This means that there is more than enough space for 1-2 platoons to spread out and not block LOS through or reveal each other's positions when one of them opens fire.
So, to answer your question, only the unit that fires reveals its position.