Does this timeline work?
Chase’s phone pings at 5:48 near his home, because he IS home.
He leaves his phone at home, because he has murder in mind, and he doesn’t want it pinging near the crime scene.
He leaves, drives to Fallbrook, kills the family. There’s enough time for this between 5:47 and 7:47.
At 7:47 he leaves the McStay driveway. He drives to the park and ride by I15, leaves his truck there, and walks back to the McStay home. (It’s easy walking distance, but there’s not quite enough time. We have to think that the security camera was off by a few minutes. From what I’ve heard, that’s a common occurrence.) The reason for using the park and ride would be to hide his truck in plain sight. A strange vehicle at the McStay’s home might be noticed.
7:59 on, he messes around with Quickbooks, loads bodies into the Isuzu, etc.
9:32, he phones his own phone, as part of a vague ‘establishing an alibi’ plan.
I may have some details messed up. I lean toward the idea that he didn’t have help—