Bamber himself ruled out the burning down WHF idea. There is no mention from Julie about him creating a fake burglar or random psychopath. Both those ideas have way too many disadvantages.
His only other option was to kill everyone as he did and stage the scene to frame Sheila. Then go back home and not ring the police. Then let someone else find the bodies the following day.
Disadvantages:
He usually had breakfast at WHF so would be first on the scene. Why didn't he do that, that morning ?
He still has no alibi for the whole night.
He would still be near WHF when the bodies are discovered by someone else.
If the bodies are discovered late in the day, why hadn't Bamber tried to find Neville ?
How could BW, a farm worker or the housekeeper get into WHF if the front door was locked from inside ?
If nobody could gain access to WHF, Bamber would be contacted. He may then have to get into WHF through a window. Showing he knows how to do this.
If the police didn't believe it was murder/suicide, there would still only really be one suspect - Bamber.
Advantages:
He can still use Sheila's illness. After grieving in front of the police he can tell them all about Sheila. Just as he did outside WHF prior to the raid teams force entry.
Strictly speaking he is not rounding down the suspects to two.
He had to pretend to have slept all night, so would surely not call Julie.
He would not have to allege Neville called him and he woke and answered. This claim has always been totally unconvincing.
If no one can get into WHF in the morning, he can insinuate someone tragic has happened and get the police. Insinuating Sheila. Maybe even creating a day time siege situation although that would be harder to do as Neville had not called him.
Can other people find any other advantages or disadvantages of attempting this frame ?