A close friend of mine is a baby chiro and I remember her mentioning one time that it's not good for a baby's spine/neck to have one arm unwrapped. She said that if we were ever going to do it, to swap sides so that you alternate arms each sleep to maintain balance.

I know it's against the "safe sleeping" rules... but have you tried putting her on her tummy? If she almost rolling then her neck is really strong already and the risk is very very small. Even if you pop her on her tummy and see if she'll go to sleep easier when you are with her. If she falls asleep and you are worried you can try rolling her onto her back once she's asleep. DS had a lot of trouble sleeping, and although he showed a strong preference for tummy sleeping, we persisted and made him sleep on his back. Both the girls have been tummy sleepers from a really young age, and they are great sleepers. When they were really little we found that being on their tummy completely stopped the startle reflex because their arms were against the bed and couldn't jump up. I also think they sleep better because the pressure against their tummy, face and limbs is comforting and feels more like someone is holding them.

May be worth a try