Temporarily back to the grounding issue. Normal brake fluid may or may not be conductive. I suspect not but do not know. Once becoming contaminated with moisture and residual junk it may be at least somewhat conductive. That might be an additional ground path for the semi isolated front.

Also to take into consideration is that with the brushes at the hub active both the hub and spindal are held at the same potential so there would be no arcing between them.