Update coming a bit later today.
I wanted to offer some clarification on the roles of ambassador's aides (Lenya, Castagyr, and Veer) and other characters who are tied to the city's government (Felix).
You won't be tied to the NPCs so heavily starting in the next chapter and will have a great deal of wiggle room in the performance of your duties. You will also very likely find yourself at odds with your ambassador (especially Londo and G'Kar). How you handle that is entirely up to you. You can leave the service of the ambassador at any time, of course.
From a story point of view, however, a lot of the adventure motivators, so to speak, are going to be coming from the ambassadors (especially Londo and G'Kar precisely because they play the politics game and are more than a little devious).
Regarding level advancement - I have set it up to be intentionally relatively quick at low levels but then very slow at later levels. Specifically, there are 111 "episodes" planned out. Everyone will be gaining enough XP to get to level 2 in this first episode and will be gaining enough XP to get to level 3 in the following episode. After that, everyone will be gaining 3000 XP per episode.