Indeed, but that's the whole thing isn't it, both descriptions of the Nebula are valid without hard fact.
Taranis retreating was because, iirc (I haven't played FS1 for years) the Terrans were trying to capture it. The Lucifer tracking it is neither running away nor regrouping, I don't think the Lucifer had any trouble finding the Taranis. I also seem to recall the Taranis had nothing to attack with
by that stage.
The Lucifer stopping the ancient technology is still an 'attack' theorum, as is the destroying of homeworlds. Regardless of the motivation or thought, the answer to most problems for the Shivans is offence. Destroy the Taranis, Destroy Vasuda prime etc etc. The entire Lucifer fleet act in a different way to the later Shivans, indeed, far more of what would be called 'tactics' are used by the Lucifer, which, I think led to the brain-bug attributes assigned to it
However, even after the Lucifer is gone, the Shivans are still automatically offensive, despite being disorganised and broken, they continue to fight to destruction rather than flee.
With the nebula encounter with the Lysander etc, yes there were other fighters and bombers involved on both sides, but the outcome of the combat was not in doubt from the moment the mission started, admittedly, partly that is plot device, since the Ravanna appearing is the whole purpose of that mission. But then, even the Ravanna, despite being damaged and low on fighters still continues to fight. Even the Sathanas, having lost at least 3 out of 4 of it's beam cannon, and thus reduced to 1/4 to no main firepower still continues on it's mission rather than head back for repairs or any kind of regrouping. It could be argued that the Sathy still thought that nothing could touch it, but what damage could one Sath without main weapons have done? It would have been a war of attrition, and the GTVA would have won in the end, but still it continued,