In my opinion, the type of murder/murderer is at odds with what you said. I don't see RA sharing anything about his whereabouts when he would be looking for a victim to kill there. Whoever showed up at the right time and met his criteria, was going to be the victim.
KK did not appear to use burner phones, his "ongoing communications" would have been discoverable. Since KK was the catfisher, it's implied that he told RA, in this theory, that the girls were going to be there, not the other way around. How would RA know anything about the girls' plans that day?
Phone records were obtained, they are not lost because a phone is destroyed. They simply get warrants/subpoenas for all carriers for any devices in KK's name and the records. KK's activities are being prosecuted using those same records, not just with the information on the phones. I would think if he was having contact with RA on the day of the murders and before, it would be known.