Back to the Sha”ch. You wrote that the reason for the issur of yayin shel mechallel Shabbos is because maybe he was menasech to A”Z. It’s hard to believe that he’s worse than the akum who is not oved avodas elilim.
I quote from the IG”M 8:
?”? ??? ?????? ???? ????, ??? ????? ??? ??? ????? ?”?, ??”? ?? ???? ????? ??? ????? ???? ?”? ??? ???? ??? ??? ????? ??? ??”?, ?? ???? ??? ????? ?? ????.
That Sha”ch is addressing a mumar for A”Z and doesn’t deal at all with a mumar for Shabbos.
Later in that teshuva, R’ Moshe writes:
??”? ?? ????? ??”?, ?????”? ?? ????? ?? ??”? ,??”? ???? ??? ????? ??? ????? ???.
If, as you say, it were clear that the Sha”ch was toleh the issur davka on a chashash that he was menasech, then according to R’ Moshe’s own understanding that there is no chashash on a M”S, he should have brought that Sha”ch to definitively exclude his yayin from the issur.