Didn't Picard explicitly tell Worf to pool resources between all three ships to the purpose of getting meaningful triangulation from the ranging shots, and then automatically targeting by that? One would assume all of Worf's torpedoes were targeted and fired on the basis of this pooling of combat automation systems.
Whether they were also midcourse-guided by the pool of systems is unknown, as they always missed by the slightest of margins after a virtually straight flightpath. If a straight path was almost good enough for a hit, then we cannot readily argue that the lack of curvature in the path proves that the torps didn't maneuver. They probably just didn't need
to maneuver much even when they could; high-gee corkscrewing would not have improved the results, and thus its absence doesn't establish that midcourse steering was absent.
As for tracking of impulse exhaust, it's clearly something that a starship's own sensors cannot consistently do even in ST6. That is, the cloaked ship doesn't leave a simple trail of exhaust that could be sensed and plotted - at most, it burps out a whiff of plasma here, another there, and a suitably configured torpedo has a small but finite chance of locking onto such a whiff at just the right time and place. Mostly, such a torpedo would spend time spinning in confused circles, as happened in ST6.
Not to mention that the technique might not work at all within an exotic nebula that has already been polluted by the engines and weapon fire of four vessels.