I doubt Warping in Reverse is nearly as efficient as Warping forewards, especially if you don't have dedicated Navigational Deflector Dishes mounted for your vessel to go in that direction. There are probably "Speed Limits" for Warping Backwards due to non-existent Navigational Dish emitting a Deflector field for Warp Travel, so relying on Shields is probably "Sub-Optimal".
I'd see it akin to going reverse in an automobile, you would be incredibly speed & acceleration limited going in reverse.
Possibly, but we really wouldn't know if speed and efficiency are affected going to impulse or warp backwards (or even sideways if possible).
Though, one could posit that because the Main Deflector dish (which moves interstellar objects and dust out of the way) is placed on the front of the ship facing 'forward', traveling in that direction at Warp or Impulse would be most efficient - but again, these people have complete control over shield geometry (And for the most part their own subspace field) and how its shaped (they can even turn a section of the shield/field on or off without lowering it across the entire ship... so, they can technically place a 'generator' anywhere on the ship and 'focus' the field on a given angle or vector to allow a ship to move in that direction without facing the front of the ship that way.
Technically a field like that would be like a bubble (and the word has been used before)... so, largely spherical in how its shaped but not a perfect sphere (more like a squashed bubble).
But because of the way these ships are 'designed' (aka visually represented), 99% of the time you'd see them going to warp facing 'forward'. It would only be in rare situations where they wouldn't have enough time to turn the ship around and warp away to simply go to warp backwards or sideways (and then rotate the ship while at Warp so the forward section is facing the direction in which its heading to).
Anyway, my point wasn't about efficiency or speed... but about the fact that UFP ships use field manipulation to achieve Impulse speeds (half a light speed or even slightly above that in some cases) and Warp... and that as such, 'direction' at which the ship is facing shouldn't really matter.
So, the fan animation having nacelles turning into a direction the ship wants to warp to doesn't make sense (and is completely unnecessary because they are effectively nothing more than FTL field generators and the field in turn [its shape/geometry and properties would be controlled from the bridge and would usually follow ship's overall design).
The only thing that produces a classical 'exhaust' as we understand it (and if I'm not mistaken) would effectively be the manuevering thrusters (which aren't field manipulators).
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