ooh, silly baby - you mean like throwing herself backwards from sitting?
all of mine did this and I saw other kids do it too - it is very normal for this age! I think they are doing "baby sit-ups" to exercise their core muscles in preparation to stand and walk alone.
I think you should let her 'plank', pillows, cushions, carpet, duvet or blanket underneath are all good to soften the impact.
but don't let her bang her head, she's too young to make that connection between action (planking) and reaction (pain) and she won't stop if it hurts!!!!
hold her hands and as she throws herself backwards follow her movement, then allow her to pull herself back again into sitting position.
if she can't do that, then sit her back by holding under the arms/armpits, don't pull her up, you could dislocate her shoulders or elbows!
I also used a "donut", it is an inflatable ring covered with fabric, a giant version of a swimming pool ring really.
if you put DD in it she can plank all she likes she won't fall out. unless if she learns to stretch her legs at the same time..
you could also get her a baby walker, babies love getting around in it kind of walking & pushing themselves around!
set the height so her whole foot touches the floor, not just the toes. that way she can "stand up" slightly - half suspended, half sitting.
my theory on rolling is that they don't do it cause they don't need to! when you are on your back you can see everything, so why make an effort?
put her on her tummy for play time and that will help her strengthen her arts, neck and back muscles as she is trying to look around or push her body upwards.
if you are also on your tummy facing her, you could entice her with a toy, she will probably reach for it and try to crawl to get it. reward the efforts by letting her have the toy, then start again.
worry not, she is ok!