You did see Ifield check for pulse and it was after that he went out shopping.
You also saw the wall of evidence when Roz entered the flat in her forensic gear, that's when the camera followed her eyes and the post it note was shown.
Going back to that night, events certainly took a dramatic leap when Ifield decided to go out to the DIY store and purchase the means to dispose of Roz's corpse.
I think someone else must've been in his flat with Roz before he returned. I can't see how she'd be able to rescue herself from being flat on her back, with him and a working electric saw straddling her, even though she'd have the element of surprise in her favour. That would mean she had assistance and help with the cleaning up. We know she was actively involved because she has Ifield's forensic suit and some bloody artefacts, plus his lap top and at least one phone of his.
It means that she and her accomplice kept hidden and playing dead while Ifield suited up and prepared the scene. How though did he get the sheeting under her if she was trying to deceive him that she was dead, he'd know then wouldn't he. Or, was she actually still out cold and didn't know what he was doing?
I think she got her wrist wound from the saw rather than Ifield's finger nails. I don't think she cut his fingers off for computer access either, I don't think that would work.