The split sea zone in the Adriatic is a good idea for both reasons you mentioned.
While the convoy any territory is interesting I wouldn’t be a fan of adding NO’s to convoy ability of the territories. Probably a better approach would be to add a few more convoy zones or just add to convoy ability to zones that contain a capital. Change the die roll to 1-4 or keep the same value and add a die in a capital convoy zone.
There’s always options.