yes I agree that Marcella has likely confided in Laura, Laura is very lenient towards her and also doesn't look surprised when M has he moments when in the loos.
As to why others would do that - the suspects (several of them) can all have a reason I could at least think of, but Cara had no possible motive to kill Grace or Marcella - she seems extremely straight-forward and was never hiding her sex work or stealing from anyone. Yeah she was violent to the young guy but she looked like she knew exactly what she was doing and when to stop. Of course in this series anything could suddenly come to light - but I'm basing on the info so far.
I am interested in why Grace was killed - it's not very obvious but it could either be business-related, or her mum getting wrong end of the stick re her partner, sibling rivalry etc. If Marcella is still not ruled out (hit her in her rage) then also makes sense.
But with the serial killer - he just does/enjoys it, he's compelled to do it as he's unhinged.
Yes I think Marcella is not off the hook yet - if she did hit Grace and killed her by accident, someone could have later done the bag thing and took Grace's body to the forest to protect M, someone who knows the details of investigation and the killer's style but doesn't know it THAT well so took the body away. And had to be someone who could drop M off to her house. The nice cop, Laura the boss, Henry, her H even (least likely)? Unlikely but not impossible.