I don't understand why he/they even took the body home with them? They would have known the police would become involved very soon, they'd have know they would be interviewed and checked out seeing as they were the last people to see Becky. It was obvious that the police would be knocking at the door at some point in the very near future, so why risk being caught with the body? Why risk contaminating your home with DNA and blood? If Becky was zipped up in that bag/suitcase, surely the easiest thing would have been to drive as far as away as possible and either put it in water somewhere, or hidden deep within woodland. Dismembering a body has got to be the hardest possible way of getting rid of it. As it is, I'm surprised they had as many days as they did before the police showed up.
I wonder if NM didn't realise she was dead until he got her into the house? Could he have thought she was just stunned or unconscience when he put her in his car boot? Though I can't believe he'd leave her in his car on the driveway for that long, but with this pair, who knows what they'd consider to be a good idea.