" I don't believe cavalry sat still under effective fire and that if they did they would take much more damage than infantry in the same circumstances."
Sitting still, obviously, would be your decision. There are only three ways I'm aware of, for CAV to get shot by INF.
1. The CAV was for some reason just sitting there doing nothing, when infantry walked up to within musketry range, and then the CAV sat there again and did nothing, and then the INF in the next turn's Reaction Phase got to shoot at it. I'd submit that in that case, the CAV player has it coming.
OR:
2. The CAV charged the INF, and the defending player decided to stay in a Line formation and shoot, rather than change formation into Square. That's a fairly crazy decision on his part, since he'll do at most 1DISR against the charging cavalry, who will then cut him to pieces in the ensuing melee.
OR:
3. The CAV moved within musketry range but did not charge. Given that CAV has a movement rate that well exceeds the infantry's musketry range, this was obviously a deliberate decision to place one's cavalry in the field of fire. I would submit that in that case also, the Cav player has it coming.
*
Is there some other common circumstance I'm forgetting, in which infantry could shoot cavalry?
I suppose it's possible that CAV could charge, bounce off an infantry square, and end within musketry range of other infantry units who could shoot it... but even then, due to the sequence of play, another full turn would have to pass, during which the CAV would have a chance to either get itself out of musketry range, or charge.
So there's no "sitting still," in other words, unless you choose to do so.
"If they bounce back a long way then it means they would have to manoeuvre back into position."
That's fine, but it wouldn't achieve the goal that you're setting for it: making Rallying more difficult. By getting the cavalry farther away, you'll just make it easier for them to rally. In order to do what you're suggesting, you'd have to write new Fall-Back rules as well as new Rally rules.
*
Let's not lose sight of what the game is about. Lasalle represents a "moment in battle." It's not supposed to be a grand narrative of an entire battle, complete with lulls and rest and reorganization of units, etc. The Disruptions and Recovery are just temporary, momentary things. The whole game represents just one period of intense combat and activity for a small number of units in one portion of the field.