When you say "... my baby was measuring 10 weeks 6 days . I had sex with an old friend dating back to that day," it sounds like you think if a doctor says "Your gestational age is 10 weeks 6 days," that is the same as a doctor telling you that you conceived 10 weeks 6 days ago. Do you think that, or are you aware that is not actually what the doctor is saying?
Gestational age (GA) is the count a doctor or any other medical source will use to count the pregnancy time period in weeks. But the count does not begin on the assumed date of conception. It begins on either the first day of the woman's last period that she had before getting pregnant, or on a computed first day of her last period based on the baby's size as seen in the ultrasound. This is done for historical reasons -- periods are visible, conception is hidden and unknown.
If you had sex 10 weeks 6 days prior to you having an ultrasound in which the doctor gave you a "weeks pregnant" count of 10 weeks 6 days, it doesn't mean you got pregnant from the sex 10 weeks 6 days prior. The time period to pay attention to would be more like 8 weeks 6 days prior.
So, for clarity, let's stop all the talk about "weeks" (because all it does it confuse people) and instead talk about dates.
You had your last period on April 17, right? When you say, " as my last period was April 17th," do you mean the last day of your last period or the first day of your last period?
Then you had sex on what, April 25? with someone.
Then you had sex on April 28 with your ex-boyfriend.
Then on May 23, you had an ultrasound, which didn't show a heartbeat, and now you think might have been a little off. Did they tell you an estimated due date at that time?
Then, you had another ultrasound which you explained in weeks (it was theoretically 9w6d and they told you 10w6d, etc.) What date was that ultrasound, and what estimated due date did they give you at that ultrasound?
Then you had several subsequent ultrasounds, and they presumably either confirmed your estimated due date or changed it, what due date have you most recently been given?
It sounds like you ovulated around May 1. You're asking whether the ultrasound in the ninth or tenth week could be "just a bit off." You said "then my 9 week 6 day ultrasound, my baby was measuring 10 weeks 6 days" and "So what I’m saying is, I should have measured 9 weeks 6 days but I measured 10, which would push conception back a week?" But to try to answer if it's a 'bit off' in relation to known dates such as your period of April 17 and your ovulation possibly May 1, the third piece of information needed is when that ultrasound was (not in weeks, on the calendar) and you haven't said that date.
If there is nothing else in your medical history that puts the suggested conception date into question, and if you had sex on the 24th with Mr. Wrong, then you have the question of the closeness in time between the 24th and the 1st.
You ask "can 'precum' really live that long to make it to ovulation?" which suggests that you are saying that "pre-***" is somehow lesser than real, manly "***" that might live that long.
It's not the "pre-***" that gets you pregnant, it is the genuine article, sperm, that rides into your body on the pre-ejaculatory fluid. Pre-ejaculate does not contain sperm when it is produced in the Cowper's gland. But if a guy has ejaculated recently, his urethra will have plenty of sperm in it just hanging around (it's a good, body-temp environment). If the circumstances don't include some time and some peeing, the sperm from an earlier ejaculation can get picked up by the pre-ejaculatory fluid from the erection that happens next, and carry the sperm into the woman's body. One ejaculation produces over a million sperm, there are often plenty left over. If Mr. 24th had had a wet dream or masturbated within a reasonable amount of time of then having sex with you, he could have had lots of perfectly normal, viable sperm ready and waiting, that hitched a ride on the pre-ejaculate.
So the questions then are, how valid is the guess that you ovulated on May 1, and how long could his sperm have lasted in your reproductive tract? The May 1 guess will be valid as long as the ultrasounds have each re-measured the baby and continue to come up with the same due date from fresh calculations each time (rather than merely carrying the old date forward). I would tend to believe the May 1 estimated conception date.
The length of time sperm can last has been reported variously in research. In the doctor's office, some docs will say 3-4 days, but I've never seen that short of a time listed in research, and I tend to think it's older doctors who haven't read the latest stuff who pass that one on. 4-5 or 4-6 days is seen most often in the research. Some even says it's possible for sperm to live 7 days, but other research says sperm are not strong enough to penetrate the egg after 6 days. As you can see, none of this favors Mr. 24th but non of it rules him out absolutely either.
Is the baby a boy or a girl? If a boy, along with the other evidence, I would tend to think Mr. 28th is the dad.
Let's put calendar dates in the slots again.
First day of last period was April 17. (This is for sure, right? Not based on calculations or what you thought when you found out you were 5w1d, but from some independent actual evidence, like you marked a calendar then or went out to buy tampons then and it's on your Visa card, or something. Right?)
Then you had sex on April 24 with the guy you hope is not the dad, who didn't ejaculate.
Then you had sex on April 28, with your ex who is now your boyfriend, who did ejaculate.
Then on May 23, you had an ultrasound, which didn't show a heartbeat. And they told you that you were 5 weeks 1 day GA (from the ultrasound's measurements, right? Not from you telling them when your last period came?)
Then, on June ___ (25th by your calculations) you had an ultrasound in which you were told you were 10 weeks 6 days GA, much to your surprise.
Then you had several more ultrasounds, at 16, 19 and 23 weeks, which "all point to conception on April 30." [Which is different than both your first ultrasound and your second ultrasound. Your first ultrasound would have pointed to May 1, and your second one would have pointed to April 20.] Did each of the later ultrasounds give you the estimated due date of January 22? (If so, they all point to conception on May 1.)