1. Don't know but we use Valve's Hammer

2. Again depends on choice, Hammer allows you to build them and safe as a collection (prefabs), not sure about importing though as I'm just the programmer.
3. Dunno, expand until you can't expand no mores (i.e. try and see)
4. Your code would have to manage this as a BSP is only loaded as static geometry.
5. Seeing as a BSP is one entity of solid geometry it is probably easier to create invisible collision spheres/cubes etc... that you would use to detect collision and then your code would react accordingly.
Hope that helps,
Mike
----------------------------------------------
Visit us at http://www.sunteamsoftware.co.uk