I was thinking about this episode the other day and I came to a realization. Kosh gave the answer that the younger races were not ready for immortality but the need to kill in order to produce the serum would lead to war and chaos. Thus, even if the Shadows or their allies had nothing to do with the serum, the result would be the same. Perhaps it was due to the lack of Shadow involvement that the Vorlons were willing to intervene
I took it as (we're) "not ready for immortality"
because we would very likely seek it even if it meant killing for it. Not sure if Earth leadership, or the Narn or any other powers that may have been courting Jha'dur, knew that fact about the serum, yet--but once they did find that out, would that have stopped them--or any of the younger races--from proceeding with its use? I.e. I think Kosh said we were not ready for it, because we want it too badly, and would do unacceptable things for it that would defeat any noble purpose for having it to begin with. If we gain the sense of perspective that it isn't something we want
that badly, then maybe we'd be a little more ready to have it (by whatever different and more benign means we develop to get it)--but there are also probably other factors that would make us still not ready....
But as for any angle with the Shadows, I too have wondered if there was some connection in the past between the Dilgar and Shadows. Some of the things Jha'dur says, like (not sure if exact quote) "it's the nature of the universe, that the superior control the inferior", practically scream Shadow doctrine, along with the whole "continuance of my work" thing. That they were such a power that it took many races banding together (that human-fostered "community") to subdue them, suggests that they may have been spacefaring during the last Shadow War... were they one of their allies in that war? It's a shame we don't know more about them, or this Dilgar War that obviously shaped a lot of recent galactic politics and Earth history. That the Dilgar rose and started "kicking anthills" before the Shadows reawakened would be a curious thing if they were in fact past affiliates; maybe the Dilgar were so arrogant as to think they could embark on galactic conquest without them, or that the Shadows would be most pleased when they came back and saw their initiative--or misinterpreted any "prophecy" they may have had about Shadow reappearance, and acted too soon in attempt to bring their "gods" back. Lots of possible speculation here.
Or maybe they were a race that simply developed that mentality without any First One coaching (although there was probably at least some behind-the-scenes cultural manipulations--both Vorlons and Shadows had those fingers in many pies). In lieu of any new TV productions, I wish the canon would continue to expand through books at least... and one of these should cover the Dilgar War and give some more insight into what that was all about and what motivated the Dilgar (not sure if there are any comics that cover this). Because that event, as so briefly described, seemed a pretty traumatic one to many of the peoples of the galaxy, and had to be significant in shaping more recent events. As it is, the Dilgar are treated too much as a one-off race for this episode (even if we only see the last one of their kind), given the scale and recentness of what they'd done to the galaxy. More historical reference to them and their war would be interesting.