Good news about the ultrasound, and yes, if it is gradually changing as the pregnancy progresses then that sounds just like I was. I'm sure you are fine. Have a good Christmas!
I was going to say....one side was more inverted than the other, but I can't recall now which was as it was 20 years ago.
I'm with you, read it all LOL I have experience of this, I was pregnant at 30 (I'm now 50) and both my nipples slightly inverted. They had never been like it in my life before, and have never been since either. At that point in life I was entirely unaware of bc, or looking for changes, and because it happened to both and tied in entirely with pregnancy and breasts increasing in size I thought no more of it (but did wonder how it was going to pan out with breast feeding, it was fine they came back out). The 'hollow' you refer to is something I have mulled over in the last few years.....as I do regular breast checks (I have had bc) I have also experienced that strange 'empty area' under the areola.
There are massive changes going on internally in your body right now, and the breast is going to have major glandular changes ready for breast-feeding. An ultrasound is going to give accurate results and I think you should feel reassured, but I do understand that you are jittery given that you've had a number of things to have checked in the last few years. Monitor yourself for the next few months, and any further concerns you have then ask to return to the clinic. x