No, not from April. I agree with the mid-May conception estimate, around the 16th-18th, since your first two ultrasounds were early in the pregnancy.
You do know, I assume, that the medical way of counting weeks of pregnancy includes the two weeks at the front end when the woman is not pregnant yet, to count in a time between the last period and ovulation? So, seven weeks "pregnant" or seven weeks "along" or "your seventh week" when said by a doctor, means five weeks since conception? "Weeks pregnant" is a bit of a misnomer that way, because it is really measuring a medically-defined pregnancy time period from the assumed first day of last period to the date of full-term delivery.
Implantation does not "take some time" -- at least, not the many weeks you are worrying about. After the fifth day from when the sperm meet the egg and creates an embryo, the embryo sheds its shell. Any time after that it can implant, and it had better, because by about the tenth to twelfth day of its existence it won't continue to exist by itself. If it has not implanted by probably the twelfth day since it was formed, the pregnancy will fail; it needs those nutrients.
Ultrasounds measure the baby (not implantation). In the fifth to seventh week (as counted by doctors, nurses, and ultrasound techs) ultrasound results are not very far off for purposes of computing when conception was, if they are off at all. (Maybe a day or two at most.) By the end of pregnancy, it's possible for an ultrasound measuring a baby to give a wrong date for conception if the baby has grown at a different rate than average. But not in your fifth or seventh week. Don't worry, you didn't conceive in April.
Agreed, May 18th was the most likely ovulation date for this pregnancy.