Hi
A comp layer with "collapse tranforms" checked inherits the motionblur settings of the individual layers inside the comp, so thats why the motionblur swith is non accessible.
And if you have 3d layers inside of the comp and have a 3d space in the main comp also, then when using the "collapse transform" switch the 3d layers are positioned according to the 3d space in the maincomp, and this can make them move in space.
Hope this makes sense, when reading it again i can see it sounds more complicated than it is :-)
- Robert