If an ultrasound sees the embryo and gives you a date, it doesn't matter when your last period was; once you have an ultrasound-based date, don't go home and correct what the doctor says by putting your period into calculators. Women can ovulate any time, it is not like clockwork no matter what assumptions the calculators use. Once the ultrasound sees the actual baby, that's it. Don't mess around with "but my period was ___ and so I ____." Go with what the embryo measures.
The person who said you were 6 weeks 4 days on March 22 was basically saying that you conceived on February 19. The person who said you were 11 weeks 1 day on April 20, was saying you conceived on February 16. Having sex on Valentine's Day would work, since sperm can last 5-6 days in your system (when it stops being "most potent" is a matter of differing medical opinion). Although none of this is entirely certain, it does seem to greatly favor your boyfriend.
The chance of someone from February 10 still having viable sperm in your system by the 16th is low. Unfortunately, by the eleventh week of pregnancy, there is a margin for error when using an ultrasound to tell the date of conception; it is +/- 6 days or so. Meaning, it could be as much as 6 days off. On the other hand, your sixth-week ultrasound won't be that far off, its margin for error is only +/- a day. And it is the one that suggests conception on the 19th. I would go with your boyfriend on this.
When you had the ultrasound at 7 weeks, what exact weeks count did they say you were (6 weeks 0 days?), and what day was that? (By "day," I mean, on the calendar, like April 30.) And did they give you a due date from that ultrasound? Was it one of the two you mentioned?
Hi, well, your boyfriend has the stronger odds, because you had more sex with him and he ejaculated within your body five times. This is not to say that the other guy has 0 chance, though.