[Solved] Infantry skirmishing vs Cavalry
What occurs in game is: infantry are not allowed to count any skirmish modifiers against the cavalry if the cavalry are on an attack [A] order (though they may still roll two dice and if they have artillery committed, the artillery could still contribute points).
If the cavalry are not on an attack [A] order, the infantry may contribute skirmishing points against them as normal.
The historical presumptions are that if the cavalry are on an attack [A] order, then they are closing as a mass and the infantry skirmishers all preemptively recall to their battalions. The likelihood the cavalry are fatigued on approach is low (hence the infantry Formation only receiving its dice and no skirmish modifiers).
If the cavalry are not on an attack [A] order, then the infantry would normally have skirmishers out, the cavalry would reply by sending forward small detachments to chase them down, the skirmishers would retire on their formed skirmish reserve (typically drawn up about midway between the main body of the battalion and the skirmish line), the cavalry detachments would then recall back into the main body of cavalry and reform. Due to the time covered in a turn, the presumption here is that while the infantry are likely wholly ineffective in skirmishing the cavalry, the cavalry still have to commit detachments against them, those horses need rest, etc... the result of which is probably some fatigue.
And I assume this is also the case with mixed formations. I.e. one of those Austrian advance guard formations or, say, a late war Prussian 'brigade' with a handful of landwehr cavalry also prevent an enemy formation skirmishing with them if the unit with cavalry is under attack orders.