That's not nitpicky at all, having cavitation based on screw speed at depth instead of ship speed at depth would add to realism and would be a welcome improvement, I think, to just about everyone here. (provided of course that the necessary controls are provided to differentiate between a normal and buster bell).
As for the rudder, you can actually order the rudder to different degrees from ship control, but you are still basically right. Evasive rudder orders are given from NAV, which means you're stuck with the hotkeys.
