It seems you have pretty much everything covered!
Another good use is to send them in advance of your main forces, taking military control of the regions on your way, so your armies can move on defensive or passive posture, expending less cohesion and getting faster and on better shape to their destination. Specially useful when moving the russian hordes from Koenigsberg towards Kolberg.
The russians have so many cossack units that it can be overpowered on my opinion.
A while ago we played a couple of very interesting 5 sided PBEM games of RoP gold and we used some kind of house rule to limit the cossack power.
If i recall correctly, light cavalry could only use offensive or defensive posture (i.e. take military control of a region) if stacked with a leader. But you could scout around on passive posture.
Without this rule, the dozens of russian cossack units used on their own on offensive posture+evade combat could literally "paint" enemy territory your color and wreck havoc with their supply lines.
It worked pretty well i think. The house rule applied to all sides, but it mainly affected the russians as they had so much light cavalry