For #2, I think you might be able to modify the underwater battlemap, since they're all seperated and in a convenient folder. Not sure how one would go about doing that though.
For #1, #land <province number> apparently kills everything in a province. #commander "<commander type>" sets the new commander. #bodyguards <nbr bodyguards> "<type>" sets the bodyguards for the commander. #units <nbr units> "<type>" sets units. It's all in the map editting guide.
Finally, even though this is really only the bare bones of a mod and there are no new pretenders, it's still a bit fun (I chose a Prince of Death because of the evil theme of the LA, which this presumably comes after).