I look at it rather differently myself. If two guns were used it would go a long way towards showing that Sheila did it premeditatedly? In that it looks very much as if her bed was not slept in. Therefore she could have had a lot of time preparing what she would do and one of those things would be to have two loaded guns so that she would not have to hastily load one of them a second time. Of course the same rules could be applied to Jeremy?
But this would explain the lack of residue on her hands and the unbroken nails. Because it would give her the added time after she loaded the guns and planned her tactics to shower and put on her makeup and do her nails in order to look her best.
My father in law was in the Thames River Police for a while and had to deal with suicides. He would describe one woman who had drowned herself and being immaculately dressed with all her clothes spotlessly clean. So I for one can see where Sheila could have easily done it.
If Shelia had used two .22 rifles using .22 hollow tipped bullets, when it was 4 murders and a suicide, the two guns imo would have been recovered by EP at the time!
As for the altruistic washing problem.
If, again imo, Shelia had shot them all, washed herself, did her nails etc. before killing herself, she would also have put on a clean pair of knickers - and according to Mike she was not wearing any when found.