So if on july 28th an ultrasound dated a pregnancy 7w4d and then on august 16 an ultrasound 10w2d when was conception? her period was supposedly 6/5/2021 or 6/6/2021 due date march 12th 2022?

Ultrasounds, especially early in pregnancy cannot be relied onto determine conception date or actual fetal age. They are used to estimate these points, detect any unusual findings and plan for delivery. At the infants delivery there is a better chance of providing a range of possible conception dates when actual physical findings on the infant define the length of the pregnancy in weeks.