IMO, G'Kar and Londo both had the potential for that path, and to turn away; they weren't inherently better or worse than each other, but they made different choices, from which their paths diverged (though ultimately met again). (I.e. later season 2 and 3 G'Kar is definitely a better man than Londo, but he is so because of the choices he made, not because he was always destined to be.) Definitely agreed that Londo's lack of qualifiers and more universal "great empire" request (instead of a specific "I want the Narn defeated" request) made him more appealing to Morden.

Human exceptionalism: I don't think the show as such has it, but the Vorlons definitely have a worthy/unworthy standart to which they hold the younger races. (These are the people who think nothing of destroying entire planets, no matter the bystanders, if that means dealing a blow to their enemies, after all.)