A bit risky Bamber telling Julie beforehand he is going to use MM. She may meet him beforehand & ask him.
But a good idea to use a proxy afterwards. Supporters have always said Julie can't be believed as MM did not commit the massacre. But have never said why Julie would invent MM if she wanted to frame Bamber.
Why as a proxy?
You do realise that you don't have to pull a trigger to be done for murder? In fact organising the thing ( conspiracy) is treated more harshly by the judiciary
Don't believe he was trying to " Soften the blow " So Julie would feel better he hadnt shot two little boys to death. She would still be keenly aware he had organised it.
MM was also Bambers friend, not Julie's.