On that specific example, I assumed Esplin got promoted because he was in charge of Earth. He had the most experience there, was probably the best they had, in their opinions, to keep the "Andalite bandits" in check, and the council was opening up to his ideas to take what was, at that point, where they had the highest investment. Giving him top spot gives him authority, and responsibility, at this all important site. Edriss got the promotion for finding it, but then went on to other things before they really had Earth as a make or break. Hell, given the details in Visser, she may have requested to be away from Earth, so I think the assignments are better tasked to the individual than they are to a rank.
The sub-visser system, unfortunately does not seem well thought out. Taylor was second in command to V3 in book 33 (though this is the first time we ever see her), yet in 19 Karen reported to a subvisser 19. Did Taylor get demoted, is this someone else? If she's so high ranking why is she never around before this but you can't get rid of Chapman? Tom's last yeerk should have been near those ranks, and yet there's hints the Vissers, though they can't openly kill each other may not hesitate to kill a sub-visser any more than a regular peon. What I'm getting at is the distribution and roll of sub-vissers should be more organized than we see it.