scrubbing lp with discdoctor brush and vpi 16.5

You scrub with moderate pressure. I scrub one-third of a record at a time with a back and forth motion; maybe 5-6 passes at a time, moving clockwise across the record. I judge by the condition of the record how many times I might repeat this. More soiled records require multiple applications of the enzymatic fluid. A badly soiled thrift record might require 3-4 applications. The first application should be fairly liberal and you should let it set 10-15 minutes.

Scrub the fluid then let set OR let fluid set then scrub?

Let the fluid set for 5 minutes, then scrub. It is a pain but makes a big difference.

You guys are scrubbing with vpi platter not spinning?

if the platter spins you would not have to do 1/3 of the record at a time?

Is it bad for the motor of the vpi to let it spin and scrub at same time?

I don't scrub on the VPI. I take it off. But that is only for stubbon gunk or truly dirty LP's. The rest I soak on the VPI and hold the scrub brush in place as the LP turns.