Hi.... I hopefully should be able to really help you with his. Haven't read the thread, just your initial post so apologies if I'm repeating what another poster says.
The first point is you will NEED to do a DNA test when the baby is born. Parentage is too important an issue to just take an educated punt on. Tests can be done discreetly if you are not wanting to let either father know.
Secondly, it is possible to date a pregnancy (and thus conception) with relative accuracy I.e. You conceived plus or minus 2 days either side of x date. But this DEPENDS ON HOW EARLY You had your first ultrasound.
Generally, ultrasounds done at 7 weeks are the most accurate, capable of pinpointing conception to a 3 day window, which, in your situation would give you the clarity you need.
A 12 week ultrasound has accuracy of plus or minus a week. In other words, when you count back the correct number of days from a 12 week ultrasound (and then count forward 14 days as I am assuming you understand medical professionals start counting a pregnancy from the first day of your last period so that conception actually occurs 2 weeks after that date)... The date you get is x date and your conception window is one week either before or after that date (if going by a 12 week ultrasound). Quite a wide window. At 22 weeks the conception/ due date can be out as much as 2 weeks.
It sounds to me like your midwife has told you that at a 12 week ultrasound your dates could be out by a week either way. This is correct...but your earlier ultrasounds should be much more accurate and almost spot on if you had a 7 day ultrasound.
If you send a reply and set out the dates you had each scan and what the scan dated the pregnancy at e,g, scan 15 November pregnancy dated at 8 weeks 3 days, I could then try and work out for you the possible days you conceived and hopefully rule out the guy you are worried about.