I don't exactly follow (but I'm the sort of person who can only think in diagrams/pictures) but if it'll work for you, and if you don't want to break down the sump...another way to add a divider is to take egg crate and put fiberglass window screen on it...of course that lets water flow through, but it can be used as a barrier against getting substrate into pumps or keeping fish/inverts out of part of a tank...and it can be done in a wet tank...just more food for thought...