This is indeed a planned feature. Instead of merely giving you a cursor, the plan is to allow you to draw lines and curves freehand, superimposed on the map (but not limited to being inside the map). Remote players will see the cursor as it moves and draws.
It will work much like chatting; you'll be able to hide scribble (similarly to how you can hide chat with PageUp). Scribble will be recorded in replay files, and once a game is over you'll be able to see your enemies' Team Scribble in playback (as well as your own team's, of course).
It wouldn't make sense to use the underwater sprite unless the Concrete Donkey splashed and sank more slowly once it hit water.
The Donkey is so huge that its top half disappears suddenly once goes below the bottom of the ocean and is deleted. It is deleted "too soon"; with slow sinking this would become much more noticeable. I think this might be why Team17 made it fall off the screen instead of being affected by water.
Having said that, it's possible to make an exception for the Donkey and require that it sinks further below the ocean bottom, so that its sprite can scroll completely out of sight before being deleted.
This should be easy to add in the new scheme format.
It's much better to go straight to the source of the issue. In this case, you want to avoid automatic worm rotation in a Rope Race. But the reason you need multiple worms in the first place is just to safeguard against being dropped from the game due to losing all your worms to drowning.
Automatic respawning upon death (as a scheme option) will solve this.
Is there any reason to add a "Last used worm" option if there's already a respawning option?