There is nothing to say Sheila was left lying, apparently dead, with a rifle across her chest and in the kitchen! As Mike says, the initial wound could have been been sustained quite a lot earlier - what is certain, is that this first shot did NOT kill her - it merely stunned her.
There has been a lot of debate around the original wound having clotted over. I don't know for sure whether there were in fact 2 bodies in the kitchen - all I can refer to is the police report of 2 bodies in the kitchen. There is absolutely no way 1 body could be mistaken for 2 bodies. The recording of the 2 bodies clearly states '1 male and 1 female'.
Also, don't forget that the police did NOT initially treat the scene as a murder scene, which is why there was such great contamination of evidence.
Can I just clariy this ridiculous theory of Jeremy talking about buying a porche - he did NOT say he was buying a porche he spoke about a sports car kit - don't forget, that at that point, he would not have known his family had been wiped out.
Jeremy was at that point in his life, young, naive, stupid, conceited, selfish, but all that does not make him guilty.