On the topic of learning expressions from screenshots, here is an archive of B5 screenshots, if you're interested, sorted by episode so that you can avoid spoilers. I've found drawing Londo in particular to be really good for practicing complex expressions, particularly in some of the later seasons, since he tends to have at least three different expressions at once.
Also, the way I've found to capture the expressions the best is to paint them, not draw them, using the technique that you call pure paints. I find it a lot more fun than trying to get the face right with just lines, and it makes you think more about the structure and shape of the face, rather than just what lines are in it. Here's an example of what I'm talking about:
Ignore his outfit, I never really bothered to finish it.
I generally take 2-3 hours on these, over the course of a couple of days, so you probably won't be able to go into a ton of detail, but I do think that it could be good for you to try out.