Contacts unreliable but X and Z moving in sync

My idea of using electrical probing to find a conductive surface doesn't work. The probe bends visibly before my conductivity meter beeps. But using the small USB microscope to observe contact worked OK. Any electrical experts out there feel free to suggest solutions that don't involve wetting the contacts with mercury or gold-plating everything.

So I have distance indicators on the big microscope stage. I used that to measure how far the probe moved on the Z axis when the control panel moved the mechanism by a theoretical 2mm. It moved 1.39mm +/- 0.01mm. Then I did the same with the X axis, which tries out all the trigonometry code. It moved 1.4mm +/- 0.05mm. Amazingly, actual movement coincides on both axes! If I set the GRBL steps/mm scaling right, we should be nearly there.

One problem clear in the microscope is that the probe wobbles in the XY plane when moved in the Z plane by +/- 0.05mm approx. I don't know if this is inherent to the delta stage design, or if I've stuffed something up. It might be due to the huge offset probe arm. We shall see.