It was not done to frame JB otherwise they would have arrested and charged him immediately. It was done to cover their arses so that they could tell any future external investigators their scenario of 4 murders and a suicide, hiding the accidental shooting.
And, if this WAS the case, you have to admit they've been remarkably successful.
There was absolutely NO need for them to bring JB into the equation. They had their 'culprit', complete with murder weapon, and, conveniently, dead. She couldn't protest her innocence. MORE!! Allegedly, they had her suicide note. Surely its reveal would cut through -and out- anything the family had in the way of suspicions about JB? But they chose to keep it hidden, causing the whole Essex force to lose face and clocking up thousands more man hours at vast expense to the tax payer, and for what, exactly? To cover up a shot they had every right to take in the line of duty!!
The saying goes, that once a lie is told, other lies become necessary to explain away the first lie. Thus it seems with this convoluted theory of yours, the skeins of which have become more entwined as attempts are made to make something from nothing.