You've got heaps of great suggestions here. I just want to add: have you tried a wedge pillow under her head so she is on a bit of an angle? Also does she like sleeping on her tummy? When DS was little he liked best to sleep on his tummy, so during the day I would have him where I could keep an eye on him and let him sleep on his tummy. Might be worth a try?
I hope you get a breakthrough soon.
I would probably go more from the adjusted age than her actual age.
HTH, you are doing a great job.