My ultrasound on July 22nd is measuring at 6w5d at 0.74 cm. What is the absolute earliest I could have concieved? And how accurate is this? Thank you!

Life is not exact. If a prenatal dating ultrasound says 7 weeks pregnant, that estimates the conception (fertilization) date at 5 weeks ago, and the start of the last menstrual period at 7 weeks ago. This usually does not matter, as women generally have sex with only one guy when trying for a baby. The estimate can be a week off (life is not exact, and surprises happen), so fertilization may have been 4-6 weeks ago.
~7 week ago. Ultrasounds are most accurate for dating a pregnancy early in pregnancy, so your calculation is probably accurate +/- 2 days, assuming it was performed by a certified ultrasonographer or experienced clinician. You likely conceived around 61/2 - 7 week ago. Take care!