I worked for VDO locally for a while.
A small flat blade screwdriver with blunted off edges is what I use to remover the bezel. Work slowly around and gently pry the edge off the case.
We always used the round blunt handle end of a screwdriver to roll around the bezel when re-fitting.
If you feel the need to clean while it's apart, then do so gently with a soft lint-free cloth........gently.
Calibrating a mechanical speedo involves checking the "count" over a set distance, putting the speedo on a calibrated drive using the count recorded, then adjusting the mechanical spring tension of the meter windings until they are accurate. Inconsistencies high to low deflection can be adjusted by "tweaking" the magnetic cup that surrounds the windings.
So there you go!