A correction to make regarding the skill shot: Maybe it's changed since the last time I played it, but the three white inserts on the plunger lane correspond to the three toprollovers: Top to left, middle to middle, bottom to right. The rollovers that become lit are based on which of the inserts on the plugner lane were lit when the ball crosses the switch and enters the playfield proper.
There's a 6-step sequence as to which ones are lit: All three, then bottom only, then middle only, then top only, then bottom only again, then bottom and middle. This means the right rollover is the one most likely to be lit for a skill shot, though you'd rather time the plunge so all three are lit. To my knowledge, there isn't any additional benefit to having fewer than all three rollovers available for the skill shot: You get the same amount no matter how many or which ones there are.