1) With regards to the sergeants, each of the sergeants generally has one specialty that they perform better than anyone else. Jalboun is best assigned to Special Missions. Light of Heavens does a great job in everything, but she provides the biggest boost to recruitment and training out of all the sergeants. (However, Training is largely bugged and so it's mostly worthless assigning LoH to training.) Bevil is unique in that he can be trained to patrol the roads AND improve taxes collected. However, unlike the other sergeants once he's assigned to a role he can't be reassigned. If you're trying to maximise your income, it's best to assign him to taxing the merchants. Otherwise, I usually assign him to patrol the lands because Land Security deteriorates faster than Road Security.

Katriona is basically the all-rounder who does a decent, but not great, job at everything. I usually stick her in Special Missions in the early part of the keep re-construction, then re-assign her to Training or Road Security.

2) I can't really list all of the different Greycloak options, but basically, the more Greycloaks you have and the better their training and equipment, the more effective they'll be at whatever task you assign them to. The better your Road Security, the more income you'll make from taxes from merchants. The better your Land Security, the more income you'll make from taxing the peasants, and the more recruits you'll gain.

3) The special missions are pretty much fixed, although some of them will only trigger once your Keep reaches a certain size, you hit a particular threshold in Land/Road security, or at certain points in the story. Note that the last 3 special missions are notorious for being easily bugged and never showing up, although by that stage in the game you're probably not missing out on a lot by them not happening.