haifa i “think” youre right that the Poskim say that everyone should do mechira.
im not sure your logic is exactly correct though.
do you sell chametz that you dont know about? im not sure but i believe you only sell that chametz of which you are aware. we rely purely on bitul for the chametz we dont know about (again, i think)
the blios in the kailim. certainly they are included in the sale, but in truth i believe that’s just sort of thrown in for good measure, does anyone actually want to buy the blios. just try someday to sell them to your goy neighbor, how would you even extract them from the pots to sell them. how could one make a kinyan on them?
i think it may be complicated but your question to charlie is a good one. id be interested in the answer.