I think you've hit a lot of it right there. Problem is, this book & this series are marketed as YA. They are shelved in YA. Yeah, there are a LOT of adults reading it, probably just under a third of the audience. But they are not the intended audience. And teens don't get that concept of maternal love, as you said, and apparently many are NOT interested in getting it or reading about it or having it be the focus(perfectly natural & understandable).
Meyer wrote this book for herself, as she has said, not her audience. In that way, she let her audience down in some respects. We as adults may love or hate it based on our perspectives, but I feel bad for the teens who bought into the series and had it switched in tone, development, and plot at the last minute.
Ah well, she is crying all the way to the bank I am sure. Everyone has a fall if they rise high enough--she will get right back up again, no doubt.
I loved it but I agree BD was not YA at all! I went to the release party here and most of the girls in line were probably around 13 and 14 yrs old and as I was reading the book I gasped a few times thinking that the book was way too mature for that age group. Even though I loved it I felt like it was taking place a few years after Eclipse not a few months!

Each to their own I guess...but I feel completely content and happy with the way the series ended!!
I don't think this will affect Midnight Sun at all, it's a completely different story than BD was.
I loved the Deathly Hallows, btw.