So the answer to the op's "just adding an extra track judiciously along the WCML ..." is yes you can, and the result of a 'judicious' track addition is to separate it from the clutteer of the existing main lines as much as possible, including across (relatively) low-cost farmland with judicious use of tunnels and cuttings. In other words, this topic has been throughly thought through by the HS2 planners, then surveyed and engineered to create the current judicious route. It has also been debated numerous times on this forum, each time the consensus has been the same, i.e. the route that will be built is the only practical one that delivers all of the benefits.