So, should all clad coins be precoated with ferrous sulfate?
MM's explanation is satisfactory for the Kennedy half. We can speculate that if it were dug from common acidic soil, an initial corrosion rate of 0.005mm for the first year or two is reasonable, and approximately 0.002mm per year thereafter for 75-25 copper nickel clad alloy. Apply these rates to the thickness of half dollar cladding to get the number of years of burial necessary to sacrifice almost all the nickel. [See. "Copper Alloys in Seawater: Avoidance of Corrosion," R. Francis, CDA-Publication 225 - February 2016.]
MM just does not have the chemical background to accurately explain his "... 'exchange of copper' or being 'covered in copper molecules' which sounds like a much vaulted 'molecular copper effervescences' process" he keeps mistakenly referring to.