Hi Navin,
Some very good questions indeed. First the Bagua concept on the torso is basically the same idea as The Bagua concept, or stategies of stepping. The diagonal angles refer to the Four Horizontal golden points of your opponent. ( think two shoulders and two hips, left and right.) then the other sides refer to the minor blocks. (up, down , left and right, think head, groin, spleen, and liver( for reference of position))) Basically you are putting the bagua symbol in front of your opponent's torso. Both repel and cover techniques work for your two upper golden points, while follow carrying and hook leading work for your lower golden points. the movements from there, or strategies are up to you and/or your opponent.
Remember White Crane is pattern based, so you may lead into Chin Na's, wrestling, punching, kicking, etc, etc.. from those blocking patterns.
As far as the stepping strategies, well at the last seminar, Master Yang actually started talking about it. He said there is actually a chart out there with a code for actual strategies but he doesnt know where it is, partially because it is supposed to be one of those secrets not to be released to the outside world. This stepping would of course lead you to the strategies of kicking.By the way, The kicking patterns are shown in the back of the White crane book as well as the White Crane Video 4 yet to be released.
As far as the Wing Chun system, or Yongquan, relaton to White Crane. Many of the ideas are common because that is where Yongquan's roots are. It took out alot of the more complicated body motions of white crane and became more simplified.( disclaimer here- as far as I know)
I am sure there is more info than I gave but I hope I have answered your questions to some extent.
If anyone else knows of the bagua stepping charts out there I would love to know. Even More important, Master Yang would love to have it for translation.
Hopefully we will see you at a White Crane seminar in the future?!