It's an archeological date, not the exactly "oh-it-is-now-produced-and-we-use-it" date!
But most other earlyer archeological contexts are not really "saved", the earliest save one is this one at Kalkriese/Germany.
Maybe the segmentata was used in the civil war as well, but we have for now no archeological evidence for it.
I think it would be known if it was used in the civil war as it was well documented and there have been quite a good archeological finds for it.
I'm not sure. Well, you're right, it is well documented in writings (De Bello Gallico & De Bello Civili), but more in structure and tactics, not about the specific equipment. And note, both are written by a single man (or the view of a single man), so we can be sure, that he didn't write the whole truth! We have only rare findings of roman military for the late republican period. To be honest I can't point out a single evidence now. Doesn't mean there is none, but I don't know any specific one now! I'm not so familar with republican stuff, more with the beginning Imperial period.
The best archeological evidence for a republican battlefield overall (and I can point out now) is the area around the oppidum of Numantia in Spain (Siege of Numantia 133 BC). The best evidence in art about roman military equipment from the republican period is the Ahenobarbus relief, 122 BC. But both evidences are long before the civil war and even before the process around the Marian reformes started. And the second is just art. As we know from the Traian's column, all representations of equipment are not correct.