That must be a good sized trunk to easily fit a 5-9" woman's body in there without leaving some evidence behind - snagging some hair, fluids, DNA in strange places etc. And I don't see WHAT BC would have had on hand (like a suitcase or duffle) that would hold a tall woman's body in one piece? I know he could have used plastic, etc - but that does not always work - hair, fibers etc always seem to remain in most cases. And once she got into rigor, it would have been even harder to move her body. Plus the body would have shown what position she was in at or shortly after death - curled up, on her side, back, etc.
BC would have had to fold Nancy up to get her into the trunk and then unfold her to get her out - from an awkward height. I can see a possible body dump by one man with an SUV, much less so with a BMW sedan trunk.
My Opinion