I think you will find answers that you can have confidence in. Your odds were not high of it being a disaster, but since it would have apparently been a disaster to some extent if things were not as you hope, you are smart to test.
(Be sure the fee *is* not merely for your local lab to draw the blood. You don't want to be stuck for further fees from the lab to whom they ship the samples for analysis.)
Your March 8 due date gives you an estimated conception date of around June 15. But since your ultrasound was in your 8th week, it will have a margin for error of +/- 2-3 days. This means you can't rule June 17 totally as the day you conceived. That means you have to look at how even your months are, do you have a period exactly every ___ days? (Like, exactly every 28 days or exactly every 30 days or whatever?) Or is it more random? I have heard you say that Partner 2 did not enter you but touched the outside of your vagina with his penis. Cannot assure you that this was enough to protect you from getting pregnant but it definitely makes it more difficult.
Was the event with Partner 2 around when you might have ovulated? Have you had an ultrasound yet, and if so, did they give you a due date based on the measurements of the baby?