Totally understand your fear. YANBU.
You need to talk to the midwives about your previous experiences as soon as you're in the Maternity Unit, they won't be reading your notes so you have to keep talking. Is baby's dad going to be there and can you count on him to be assertive too? I don't know how I would have managed without DH pretty much becoming my voice. I asked for drugs and kept being told 'soon soon' but when DH said 'she needs something NOW' I was off my head within minutes! 
Writing a plan is a good idea, BUT I would see it as a script to keep repeating rather than a 'contract'.
As lubeybaubley has said (and I'm sure you're aware of) the weight scans ARE unreliable, so you may be getting yourself worked up over something that won't happen. However, if your baby IS a big one, you're going to need to find your voice early. (Mine was estimated at 10lbs, he was 8lbs).
I can't really think of anything else to say that you don't already know, YANBU at all but you need to try and control your imagination and terror, try not to focus on 'what ifs' - one way or another your baby has to be born. I mean that nicely, but that's the reality.
Have a virtual (((HUG))). As far a PTSD, you can't stop it if it happens, but you know the signs, and can prepare. Take it EASY after the birth, have family on hand to give you as much peace as possible. Don't be a martyr mum and try to do too much afterwards, I speak from experience. Go at your own pace, cry, rest, talk. I spent a week in bed after the birth of my second child, baby next to me, DH and family doing everything else, and I swear that it was my stubborness to do as little as possible that saved me from being at the mercy of PTSD for long. x