Hi!
I'll take a stab at some, but Scream may over rule me on some.
1) Good question. The idea behind it was with standard armored vehicles in mind, since in reality tanks are vulnerable from the top and back. I would not apply this to knights or titans, no aircraft. Since tyranids are living creatures, I'd apply the rule depending on their role. If its a "tank" like creature then apply it normally. If it is a titan/knight creature then no.
2) No, they are mechanical.
3) I believe your interpretation is correct.
4) Just one model. You are correct that the old ability was over powered, especially against low morale armies like Orks and IG. Used correctly its still a pain, but not as overwhelming.
Primarch