Hi hon,
(((Hugs))), that sounds so tiring. Just like the other thread you posted on, I wonder if your LO is in some discomfort. I'm sorry your doctor has not been much help. Did he or you mention reflux or any intolerances at all? Does she have an reflux symptoms?
Reflux 101 - General reflux information. When you hold her in the chair at night, does she then sleep better? When she falls asleep in her carseat, does she then continue to sleep if you take it out of the car and put it in the house?
I'm sure every day is different right now, but can you give me an example of what a day looks like? E.g. when she is awake, when she eats, and when she sleeps? I'm wondering if she's sleeping those really long stretches, she may not be getting enough feeds in during the day and that could be a factor in her night waking. Although, it does sound a bit more complicated than that.
Something like this...
W/E 7am
A
S 9-11am
E 11am
A
S etc etc