You have now answered the question and we are finally getting somewhere. (Inspector Clouseau)
"Why would this unknown killer have been trying to set Jeremy up when he deliberately left a scene which indicated that Sheila had shot herself?" That is such a good question.
Nevill is recorded as "frozen with rigor mortis but in Sheila's case they were still able to make her arm move to different positions.
So do you agree it has been proven that Sheila died last and much later than her adopted father?
So going back the killer left Nevill sitting on a chair blocking the back door. It would take a strong person to lift a dead person up onto a chair or are we to think he was told to sit on the chair and then he was shot dead.
Rigor Mortis sets in at different rates depending on several things
Room temperature — Nevill was by the Aga which was burning at a low heat, thus causing his RM to develop more quickly
RM also varies from person to person due the the chemistry of their body; certain medications they’ve taken;; plus a few other things
Sheila had livor mortis when found, which is the onset of rigor mortis.
Rigor Mortis can set in as quickly as four hour or five hours, but can take up to nine hours, depending
Whatever, Sheila was entering RM as she had mottling and livor mortis well established, so she must have been dead for at least five hours, possibly longer.