The idea does interest me, but making the bridge destroyable would be a bad idea.
HiTech
He has obviously shown interest... no need to beg.

1) Can the bug be worked out that prevents units from going under the bridge?
This is a VERY good question for HiTech. I'm sure its possible, but only he can answer how feasible it is.
i'll do you one better bud. Make 4 destructable bridges per main bridge...
This would eliminate the point of a bridge and the end result would be forces by passing each other.
Simply put the whole Idea of a bridge is to make a defensible bottle neck that promotes a place to fight for GV's.
This is a classic example of how realism many times is not fun. What is the end result of destroying the bridge. Fighting ends. No fight no fun.
That was exactly why I suggested only 2 destructibles. If you have a main bridge that is indestructible, then 2 destructible bridges, the 2 destructible bridges provide a flanking option, but you'd be largely unprotected on the smaller bridges, and the bridges could be blown up, possibly while you're on them. The key here is layout.
My suggestions summarized below...

I am also all for PT and LVT spawns.