This is the third fishing-expedition thread that you have started. In the first one that you started, some books were recommended to you. Have you (a) obtained, and (b) read them yet?
All of the various parameters that you are asking about, cannot be selected in isolation from any of the others (as you are trying to do). If you examine front suspensions of various vehicles, you will see that different designers have differing design philosophies in this regard. That doesn't necessarily make any of them right, nor any of them wrong.
Scrub radius has more to do with steering feel and feedback than anything to do with oversteer or understeer. Zero scrub radius will give minimal "kickback" in the steering when striking a bump, but also minimal feedback to the driver. Positive scrub radius will make the vehicle kick (steer) towards the side striking a bump. Negative will kick it away from a bump. If the vehicle has diagonally connected braking circuits and one brake circuit fails, negative scrub radius will help keep it stable. If the vehicle blows a front tire at high speed, negative scrub radius will help keep it stable. If the vehicle has the front wheels driven (all-wheel-drive counts!), the scrub radius will have to be small to avoid excessive steering effort. This is trending towards having a slightly negative scrub radius ...
And yet most traditional rear-drive vehicles have a somewhat large positive scrub radius. But not all of them. I own a rear-drive vehicle that has no all-wheel-drive variation, and it appears to use a slightly negative scrub radius up front.
King-pin angle ... As little as you can get away with, is a not unreasonable approach. The stuff that needs to fit inside the hub is probably going to put limitations on where you can put the ball joints. High king-pin angle sends the camber of the outside front wheel in the wrong direction at high steering angles. VW likes using high caster angles to offset this, at least for the outside front wheel. Of course, that results in even larger camber of the inside front wheel, but that doesn't matter so much. Presumably that approach also results in high steering effort ... but that's what power steering is for.
There is no single magic bullet solution to this, that magically answers all of your questions.
