1. I don't think so. I seem to remember something about the LOS being calculated from the level shown when you put the cursor over the hex. That would make sense, since the buildings don't take up the entire hex and the program assume your guys will be peeking around the edge of the building or looking out of a window. So my understanding is that there is no upper level such as Squad Leader had. Not sure I could get that counter to stay on the monitor screen anyway.
2. Yes, I have seen it happen. I think it was against Iwo Jim. I have seen helios knocked down by Fighter bombers also. That was several versions ago and I don't know if it's still possible.
4.
http://en.wikipedia.org/wiki/Brigade
The first combined arms teams. In modern terms it varies from country to country. As a rule of thumb, it is bigger then a regiment, but smaller then a division.
As far as the helio breaking radar lock by landing, it used to be possible. I would do it all the time, give my helio a break while I tried to get enough arty on the SAM to suppress it to where I could sneak my helio off. Since 3.0 I haven't had a helio survive a Radar guided SAM or AA gun getting a lock. I have mostly stopped buying them for that reason. I suppose you could consider being converted into a flaming ball of molten aluminum filling a hole in the ground as breaking lock.
Did you get my turn today? My mail box is being weird.