“Do I have this correct:
1) Sobhana cetasika lead to good vinnana.
2) An Arahant still has sobhana cetasikas.
3) A good vinnana is not contaminated.
4) An Arahant still has good vinnana.”
Yes. An Arahant has undefiled vinnana (one could call that “good vinnana”), which is attained at the optimum of the set of sobhana cetasika, that includes panna.