I think diode temperature is the thing you're looking for.Most, if not all, diodes are fast enough to take the start of the back-EMF current.Only the diodes that turn off faster are "ready" when the mosfet switches on again.
Across the inductor.
Even with the scope isolated from earth ground?
AFAIK it's common for the scope to be grounded,
I assume your CRO is single channel?
If your scope has 'invert' on one channel, and 'add channels' mode your display will read differentially.