Jones says the marks on his back suggest he had been forced downstairs.
But he had his pyjama top on. Which had no marks.
I suspect Jeremy lifted a part of the top after all 8 shots and the beating. To make sure he was dead.
If Jeremy staged the burns then it was done with the intention of conveying Nevill had been forced somewhere or to do something- mostly likely to convey he was forced to make the call to Jeremy. He could have hoped police would think that Sheila forced him to make the call so she could kill Jeremy as well. Jeremy presented such a scenario to Ann Eaton.
If he did stage the burns he clearly did it after Nevill was dead. He was already lying down with his shirt pulled away from his body.
Another possible reason for staging them could have been to make it look like Sheila was going crazy and torturing him.
Instead of being staged he could have done the burns for practical reasons such as to test whether Nevill was dead or faking.