Must admit, i really enjoyed the series and hope there is a second one. Although i did have to sit and ponder a few times throughout and here are my thoughts;
Whilst we don't know what caused Marcella's 'blackouts' (or should that be blank-outs, absences of memory?) but we do know that they leave her prone to bouts of extreme violence. So, believing that she may have killed Grace (last thing she remembered was confronting Grace about the affair), she moved the body. Near the end, she overpowered the killer, Henry Gibson, and probably had another 'episode' during which she put a plastic bag over his head, thankfully coming 'to' just in time to save him.
Though she spent much of the series convinced that the killer was Peter Cullen, she was the only member of the team to notice certain tell-tale signs that pointed to Henry Gibson as the murderer.
As i said, hope there's a second series in the offing.