Ron being able to tell people adds more credence to my "Bill Weasley murdered someone" theory

 

No, the Fidelius was already cast at that point, the scene I quoted is them apparating to Shell Cottage and while there Harry talks to Bill who tells him that they're protected by the Fidelius Charm claiming that he himself is secret keeper.

So unless he used the spell while Harry was burrying Dobby, the charm had to have already been cast.

 

I'm pretty sure it's exactly the moment the Fidelius is implied to being used.
Between your quote and the conversation with Bill, there's the entire skirmish at Malfoy Manor, the escape, and Dobby's funeral. That's half the night, and since we don't know anything about the casting, it might well be enough.

Bill says it's because of Ron being discovered that he sent his family to Muriel's and they can't go to work anymore. Arthur is Secret Keeper at Muriel's, not the Burrow, so it only was just cast that day. The earliest time where we're sure they know about Ron is when Dean and Luna arrive at Shell Cottage, which is just after your quote.

It's possible that the Charm was cast long before that just in case, or that Bill had cast it long before Arthur for some reason, but I don't think that's the intention, and I'm pretty sure it's not mentioned either during the summer or when Ron goes to them at Christmas.