You've covered a few points, but I have a few addendums. First of all, you're probably aware it's generally a bad idea to hold multiple baronies in a county. Holding the county-capital barony is ideal because it gives you all the taxes and levies from the mayor, bishop and baron vassals, letting you spend your demesne limit on additional counties, getting more low level vassals. These vassals don't count towards your vassal limit, because they're below count-level.
For raising levies, you can raise the whole county's worth of levies from the county holding view and then split them up and dismiss the irrelevant ones. This is done with the 'New Unit' button, which is an icon of two stick figures separated by a slash. Each holding or vassal will have exactly one indivisible unit when raised. If it's your holding, it will be called "Barony of X" (or whatever barony level title your realm employs). If it's a vassal's levy, it will be called "Liege Levy of X", where X is the vassal's top level title.
You can move units between armies with the left and right arrow buttons next to each unit. Update: Version 2.4.2 (Horse Lords) changed this into a drag-and-drop interface. Put your target vassal's levy in the new unit by itself. Then select the other unit with the check mark and dismiss it.
The opinion penalty for having raised levies too long is also per-character, like most other transient penalties. If you or the target vassal dies, the penalty will be reset to zero.
For getting a vassal to plot, it also helps to make them hate you. If they have ambitious, envious etc. traits, it doesn't take much negative opinions to see some activity. Raising levies works here, but if they're content, honest, trusting etc. it will take bringing out the big hate. You can also plot to assassinate them, invite an incompetent drunkard to the plot and abandon it as soon as they spill the beans. You can seduce, kill or request excommunication of their family members. You can appoint them to the council and fire them. You can give them an honorary title and revoke it, then give them court jester. If they're fostering someone, ask the liege of the ward to educate them yourself. There really are a lot of options for getting people to hate you...
Another long game to play with the succession is to foster their heir. They have to like you a bit to agree to this, so do it before ruining their opinion. The idea is to give the heir rebellious traits, setting them up to hate your heir. You can also fine-tune it for traits that oppose your or your heir's traits, e.g. cynical vs. zealous equals a mutual -10. If their heir inherits before you shuffle off, they'll have a +25 mentor opinion of you, which can ruin the short term attempts. As a bonus to fostering, you will have likely have several opportunities to milk them for small prestige or piety bumps by praying for them or extolling them to the court; especially praying for piety coincides with small chances of removing bad traits (usually the 7 sins).