I believe its stated that there are smaller deflectors placed all over the ships.
I don't know why you must have a navigational deflector, you guys are thinking federation ships. Think non fed ships. oooommmmuuuuuhhhhh....
Right. So obviously it can be done in the trek universe, its just not the federation way to do it. The obvious answer is that it all depends on the
holographics of the warp engines. The warp engines are creating that bubble fore and aft, so if its a bubble like bubble, you'd have to worry about objects.
But if its a needle nose pointed bubble and has some strength for integrity,
it could just resolve the whole issue by cross modulation of the engine streams in very very large and long triangles.
Not that either way is better or worse, but to some degree smaller or larger deflector grids mean that the job is being shared by those systems in slightly different ratios and the corresponding warpfeilds are thus either bubblisher or pointyisher.
Caveat. I'm a vulcan. I think star trek is great, but cannon to me is entertainment; not science.