I think that-the whole reason JB is in the frame is because of the call. If he hadn't fabricated that he may have got away with it.
I think he was concerned with providing himself with an alibi. I think he really planned it in his head.
Again, this^
If he planned it in his head, I'm wondering what his, 'damn, didn't think of that' moments were after the fact.
Why call JM at 3am on the night of the murders to say, initially, it's all gone well and finish the brief call with, 'there's a problem at the farm' or words to that effect.
If he was in a panic he would have said that first. Also, as the police said, if it was an emergency i.e. lots of blood and dead bodies he would have dialled 999 not looked through the telephone directory for the number of the local police station. His reasoning was that the police would have been dispatched from there anyway. (Not necessarily, a 999 response dispatcher would have asked the nearest available police car to attend.)
This is an indication of how much JB was orchestrating it. He's still orchestrating it now with his insistence (even he knows the game is up) that it happened how he said it happened.
That's criminals for you, apparently, they will keep lying and peddling their version in the face of overwhelming evidence.