In response to the question about how the dogs transition to tracking someone besides me -- I am not sure but they do. I think that the scent is not just a person but rather crushed grass, moved dirt and so on; this means that it is more than a person that they are tracking. They know what the start of a track "looks" like -- it has an article -- and then they just find the track and off they go. I have never had trouble with a dog tracking other people, but I do occasionally practice it before a test.
The advantage to laying my own track is that because I know where it is, I am able to read the dog as she moves through the track and can learn to tell when she is on and when she is off, and how she acts on turns and so on. I never do a map so have to rely on memory and this means that I usually know the general direction of a track but do not always remember the specifics. In this way, the track is somewhat "blind" and I have learned to trust that the dog is almost always right. Today I ran Asia and Cadi on incredibly hard tracks and I thought I had a turn in one spot but Cadi was clearly tracking straight; I went with her and she was correct, as usual.
In addition to tracking, we also worked on draft -- Cadi and Maize are doing a mother-daughter brace :)