I can measure the contact opening speed by looking for an arc but are there any other measurement methods that don't rely on high voltage ?
can you please explain how on earth you measured 100uS and 1mS just by "looking for an arc"?
If you have the coil connected to a high voltage supply, then you can't magically do something to remove that high voltage in your measurements, unless you remove the load, which automatically removes the spark anyways.
the diode across the "coil" is to reduce back emf to the MCU, not to suppress the arc created by the inductor. add a similar diode across the inductor as well, it should remove the arc issue. if that wasn't enough, add a snubber network across the relay switch contacts as well.
