hmm well a lot of your questions can be answered by searching on the net or by searching these forums, but i will answer what i know off the top of my head.

you can top a plant whenever you want, it all depends when you want the plant to start bushing out, basically. when a plant is young, it will generally just grow upwards with no sideways branching for quite awhile, so if you want to start branching earlier, you can top it sooner.

you can take off as much as you want; you can either just pinch off the very top of the growth tip, or you can cut off down as many nodes as you want...doesn't really matter. i think where you top it (like if it's done at the 6th node or the 10th node) affects how many sidebranches will form, but im not sure. it is a GREAT idea to use the cut off part of the plant to use as a clone. it would be a waste just throwing it out.

i think your point about yield is currently debated among growers. i think the benefit of topping when it comes to yield would be from getting the growth tips more light=bigger buds.

hope that helps a bit- but the more you research about this the more you will find out. these boards are invaluable.