It is difficult for us to find out the root cause of the problems, as sometime the H2O2 and dummy plating cannot solve the problem. The pre-treatment process is effective and I believe the black spots on the surface are not due to the dirt attached on the raw material which cannot be removed by the pretreatment process. The treatments that you are putting your bath through, may or may not be solving the problem. Rinse well and rack in the nickel tank. If black pits occur the cleaning in your tank can be assumed to be good.
Pumice is an excellent manual cleaner, but too inefficient for use on a daily basis.
The peroxide adds you are making could cause the problem if residual peroxide exists in the solution. Heat the solution to about 160 deg. This will decompose any excess peroxide that MAY be in the solution. Stop here if the problem goes away. If the problem is organic and you know that a carbon treatment is needed.