Surely Marcus's point about saying Abi could have murdered Lucy is to question the whole premise on which the police have built their case against Max and to show the jury that there is reasonable doubt that he did it as it could equally have been someone else. Lauren should be pleased with this line of questioning as it is designed to get her father off without implicating Bobby in any way.
In any case, what right has Abi got to be so self-righteous? She's the one that dobbed Max in it in the first place? Why shouldn't he fight back? And why Lauren should take Abi's side over all this when she knows the truth is completely beyond me. Bring on Brian Rix and Coco.....