How about providing a reselect command to reselect the previously selected note. This would make switching back and forth between keyboard and mouse rarely necessary. This would be eespecially true if up/down arrow moved selection to the staff above or below. In general great work to all.
Because selection/deselection is part of the undo stack, you might sometimes find that Ctrl+Z restores the last selection, provided you do it immediately after the operation that caused the selection to be lost, of course.
But I agree that it might be a nice idea to have a means of restoring the last selection even after another operation has taken place. I’ll definitely think about this.