I do wonder if G'Kar told himself he was going to do something more sensible as a test, but then couldn't resist once he was high as a kite because he just has to know.

On the one hand, sounds plausible to me, on the other, well, if he had told himself "I'm going to try this out myself" he had to consider whose thoughts he'd be reading in order to check whether the stuff worked as advertised. And as the Judge pointed out, he avoided mindreading every by stander and went straight to Londo's quarters. What I mean: if G'Kar had been in Down Below when he took Dust and then reconsidered once he was high, I'd believe he didn't hit on the idea until he had taken Dust. As it is, I think he either knew he wanted to do it before, or was repressing what he wanted to do to the degree that he didn't even consider he needed a guinea pig in addition to himself.

I will never be over the fact that not only G'Kar's rebirth as a prophet but also every single development in his relationship with Londo from this point forward turns on the worst thing he ever does.

Very true. I can't think of another show that managed to pull off something similar and made it feel right. Another thing: the fact that Kosh bothers to intervene and stage a vision for G'Kar is quite a change in Kosh, who wrote off both the Centauri and the Narn in s1 as "dying people". (And I think that's the general Vorlon attitude. The Narn are no longer of interest because there are no more Narn telepaths, and the Centauri because they're considered tainted now if they weren't before. The Minbari and the humans are needed for the war against the Shadows, so they are the future. But Kosh - this Kosh - in his years on the station has started to care about individuals and people beyond "are they useful against the Shadows", and that's one big proof for it right here.

G'Kar is also egocentric enough to think all those times Londo skived off to drink or gamble or watch strippers were meant to annoy him, personally, not the result of Londo and the Centauri not taking the project seriously.

Good point, and Born to the Purple is a good example of this, because for once, Londo really does nothing there to spite G'Kar or because of G'Kar, he hardly thinks of G'Kar at all, it's all about Adira both in the positive and the negative sense. But to G'Kar, it's a personal slight what's going on.