5) is also somewhat game-breaking, in that i've ended up with hundreds of thousands of prestige. How? Once you take over a king title (or, say, the HRE, with 3 king titles in the bag), and vassalise the former of said kingdom... he probably won't just submit to your rule easily, but will rebel and/or declare independence eventually, and regularly.
When you force him back to the fold, you'll get the warscore from both of you having claims on one or several king titles, and if you don't force him to relinquish any, the prestige, which, as I said, can be hundreds of thousands from one war alone. There seem to be diminishing returns on this method: once you have tens of thousands of prestige already, you don't get the full war score minus peace score amount in prestige anymore, but that doesn't really matter, since i'm not aware of any difference to game mechanics if your prestige is 1000 or 100 000 or 1 000 000, as long as it's 1000 (+1% vassal loyalty, etc). Except for the game standing that determines who wins (F2).
I don't actually this that gamey until you end up doing this all the time, for generations, since it's more of a side-product of the military conquest of an entire kingdom. I guess I should force vassals like this to relinquish at least 1 king title or something each time as a role-playing rule, to eventually diminish the massive prestige gain from completely overwhelming the great-great-grandson of the triple kingdom.