I don't think you folks are hitting on all cylinders. There are only two alternatives for assassination motives: (1) Kosh is actually the target and then one has to examine the usual motives for attempted murder; or (2) the assailant is trying to incite the Vorlons, whether to destroy Babylon 5 or to declare war on the EA.
The second alternative is not without its problems. If you organize a conspiracy to accomplish a military goal, then you have to have some reasonable belief you can control most of the outcome. Deliberately picking a fight with the strongest power in the known universe (remember at this point only a few Minbari even know about the existence of the Shadows) is the height of stupidity. G'Kar may be driven by revenge but he isn't stupid.
Next, we wouldn't even know there was an assassination conspiracy if it hadn't been for Lyta's scan. The viewer is made to think as the plot develops that Sinclair was the target of a frameup, but if Lyta doesn't scan no one would know of a frameup. (This action was more likely just a backup plan for the assailant to avoid detection in case there was a telepathic scan, not because he expected a scan.) Moreover, as Dr. Kyle pointed out, the Vorlons seemed content to let Kosh die by ordering no recording and no opening the environmental suit. If Kosh dies, then, the whole notion of a conspiracy to incite a negative reaction by the Vorlons falls apart. So, unless you make Sinclair and/or Dr. Kyle part of the conspiracy (Sinclair ordered Kyle to open the environmental suit and Dr. Kylce came up with idea of the scan), there is no way for the conspiracy to succeed. And, Dr. Kyle's proposal for the scan was based on the fact that he got the green light from Sinclair to try to save Kosh's life. But, thanks to the scan the Vorlons do react and go ballistic.
G'Kar, of course, takes advantage of the whole situation for his own political and military ends, but I conclude he was not the leader of the conspiracy. While arguments from silence are problematic, my theory is that the Minbari assailant somehow learned of Kosh's involvement in advising Delenn at the Battle of the Line and in part blamed him, making the attempted murder much more personal. He obtained help from the Narns, but events took a different turn than he planned.
QMCO5