About the prerequisites.
I think it might be a good idea to have a certain level of observation.
It's not unreasonable, but since it is debatable, I'll leave the final decision up to you.
It would be insane for someone to be very good at tracking with little to no observation skills.
Maybe you could have it so to be competent/expert/master at tracking you must have #observation.
That could be put into the skill development section.
As for wilderness survival and land navigation, they would most probably fall under related skills.
One can track in cities, thus eliminating the need for wilderness skills.