Why is the interconnection between test probe and coupon so important? And would it be possible to use a test lead to allow the ground to be connected more easily?
Controlled impedance measurements are only accurate when the test system is itself a controlled impedance system right up to the end of the probe! Poor interconnection i.e. long ground leads or coils of wire will cause reflections which will reduce or invalidate the reading from the track you are attempting to measure. A wide variety of probe footprints are available, and for occasional use there is an adjustable probe IP 50-V.
