Counting the Thirty Days When Shiva Was Observed After the Festival

If someone's father passed away during Chol HaMoed, from when do the laws of the thirty days count: from the burial or from when Shiva was observed after the festival?

The thirty days are counted from the burial, 30 full days, and the eighth day of the festival or the seventh day of Passover is considered only as one day (and if Shiva was observed before the festival, Shemini Atzeret is counted as seven days).

It is stated in Moed Katan 20a that even if the burial took place during the festival, the festival counts towards the thirty days. Shulchan Aruch, Siman 399, paragraph 2: "One who buries his deceased during the festival in Chol HaMoed observes the laws of aninut until the burial; after the burial, he observes only private matters, and the festival counts towards the thirty days, completing thirty; even if buried during the festival of Sukkot, we do not say that Shemini Atzeret cancels it." And the Hagahot Maimon wrote in chapter 10 (DK 5) that Shemini Atzeret counts only as one day if buried during the festival. However, the Maaseh Binyamin, Siman 71, and the Maharshal, Drisha, and Shach hold that Shemini Atzeret is counted as seven days even if the death occurred during the festival, but the decision of the Mishnah Berurah, Siman 548, SK 4, is that Shemini Atzeret is counted only as one day if the death occurred during the festival.