An unspent ejected shell would show the ejector markings. The ISP Lab performed testing on that bullet and said it was a match to RA's SS according to the PCA. It also stated that ballistic science could be interpretive and depended upon the expertise of the examiner. I like to include full disclosure when stating something like this. Dueling experts for and against is my guess come trial time.
I think RA held the gun on the girls until he got them where he wanted them and probably had them tied up with something. He wouldn't have needed the gun any longer so he could have easily killed the girls one at a time with the knife.
Libby's cell phone being found underneath her according to the PCA and found underneath Abby according to the D's Franks Memo had me really thinking. I believe Libby was the main target, so he killed Abby first and spent more time focused on Libby.
How and where her phone could be like you said, it fell out of his pocket in a last minute rush to get out of the area or Libby was smart enough to hide it under her. Typing this out has literally given me goose bumps. How terrifying for these young girls. I am so very sorry they crossed path with this monster that day.
#Justice4Abby&Libby
All JMO