"A tunnel of bent space stretching in front of the ship" sounds cool but isn't consistent with how it's done in Trek. Warp fields don't extend - they just surround. And indeed if one wouldn't need to stretch space all the way from A to B into a wormhole, and an extension of X ly would do the trick, why shouldn't an extension of two millimeters be fine as well?
I think maybe I'm too thick to understand how warp fields are supposed to work and the visuals don't really help. I understand that you're not compressing the whole of space so you must be compressing only a small area around you and extending that in front of you but you have to keep extending that field forward to get anywhere - after all, your speed is zero, it's space that is being moved - so it's not a tunnel but more of a scrunchie isn't it? It sounds to me that this would be dangerous to anything in your path and two ships should never be able to interact at warp at all unless it's in the unlikely event that two exactly aligned warp fields cross and that sounds like a death sentence if the bubbles really are that small.
I would have thought that warp drive would have automated systems that drop ships out of warp if another warp field or an object too large for the navigational deflectors is detected in your path or automatically adjust course perhaps?
Could transwarp be technology that allows ships to interact at warp by adjusting the warp field to avoid disaster - a failed experiment according to Excelsior's history but successful enough to fit it to the Vengeance?