Questions
-Is there any benefit in making a vassal a daimyo? If a vassal is not a daimyo they won't get too powerful (something I'd like to avoid with plots).
If a vassal of your don't have a daimyo title he himself can't have vassals, which means he can't hold more than five provinces. So if you conquer a lot of provinces you need to hand them out to a large number of direct vassals. By having fewer direct vassals and letting the daimyos have vassals under them, you reduce the number of people who vote for clan leader, which makes it easier for you to keep them happy and make sure it's your heir who gets the votes when you die.
-How does occupation work, sometimes land goes to my vassals, sometimes to me.
It dependes who controls the siege. In the siege view there is a symbol at the top left corner which tells you this (a green star for you, a blue flag for vassals). Hover the mouse over it and the tooltip should tell you exactly who controls it and will get the province.
-How does the 'sow dissent' action work? Do you also get a message if your courtier was succesful like the 'improve relations' action?
As long as he stands in the province there are random events that can fire. Once he succeeds or fails you should get a message/event from your councillor.