legal system for juveniles (California)

The problem of there being a lot of ways to go is that there are indeed a lot of ways to go. I know a lot about adult court, not so much about juvenile. I'd pick up the phone and call the DA's Office in your county and ask for their public affairs people.

In California, prosecutors can file juvenile cases directly in adult court on certain cases without going through a court hearing first to show that such a move is justified. Murder is one type of those cases, and with arson involved, I'm sure a prosecutor would do so. They call it "direct file."

The arraignment would be within 72 hours of their booking in jail. In adult cases, the defendant generally would not see a lawyer until right before the arraignment, and that would only be the public defender right assigned to that courtroom for the day. They would be assigned their permanent defense attorney a couple of days after the arraignment. My bet is a juvenile charged with those crimes would be assigned a lawyer right out of the chute.

The DA's Office I believe can file into adult court later in the process if they wish, but it would generally be done up front. It's not necessarily automatic, but in your type of case it may as well be.

The defendant would remain housed at Juvenile Hall and would get bused over to the adult courthouse -- in my county they're far away from each other.

At the end of the arraignment, the judge will assign the next dates for a few things: bail review, readiness conference, preliminary hearing. Murder cases defendants usually waive a bail review but if they don't it's about three work days later. A readiness conference is about a week to 10 days later and a preliminary hearing a few days after that. In a murder case, the announced prelim date is just a place-holder, no way it would happen that quick.

If a credible witness came forward, the police would probably get the prosecutor to interview the witness, and they'd then make a decision. In jail, it often takes 6-8 hours for the word to get to the jailers and for the prisoner to get processed out.