they stopped changing my edd after third tri due to as someone up there said, genetics can play a part the baby can get taller if the parents are tall etc... but this doesnt change how 'old' it is just how 'big' it is thats why it becomes so innaccurate.
as for the first scan thing? im very confused why the sonographer dated the baby as 2 weeks from before you conceived, you said you spoke to the midwife but did you talk to the sonographer about it? they are highly HIGHLY trained, i looked into becoming one and it was far too hard for me!
also dont forget sperm can live inside you waiting to fuse with the egg for many days
if it was me i wouldnt really want my baby at home at 36 weeks even if i was sure it was 37 as you just cant be 100% sure with these things! if it turned out the baby WAS 36 weeks its lung may not be ready and may need to be taken straight to NICU