Are you still concerned about it? I mean, perhaps you could accept that OB's offer to come on as his patient, presumably have a prenatal visit where you WOULD get an accurate ultrasound, and then choose to birth as you felt best?
Are you monitoring your BP at home? I'd have a way to monitor it at home if it's fluctuating so much and is a concern. And, are you keeping a better log of fetal movement? Irregardless of what the hospital said, I'd keep a log of when you feel the baby/how often, and if lack of movement becomes a concern, I'd quite frankly lie and tell the doctors I'd not felt fetal movement in 24 hrs.
Does the baby appear to still be flipping now that he's turned head down? I mean, he could have been flipping so much as a natural way of getting to head down from transverse, right? If he's still flipping at that approximated weight and this far down the road in the pregnancy, then that would concern me.
And, if I was sure about the low waters, fluctuating BP, I would probably want to go with the OB (esp. if you have insurance) and just avoid going in for a C-section unless you agreed with the doctor about it's necessity.
Another option I was thinking about is...traveling for your birth. What about visiting family or friends out of state in a more friendly birthing state? I recently had a miscarriage while UPing, dealt with it at home and finally ended up traveling out of state to visit family for a check-up because it was MUCH easier to find a midwife there.