She was completely on board with the FB campaign. It was easy then, they were arguing for contact with the children, she was a heartbroken nanny who missed children she had known for many years (And I agree she was probably all of these things). Once they were charged, once reports and details started coming out, then it wasn't easy anymore. Now she was someone who was complicit in her daughter's actions that day and each day since.
I do not see why the prosecution would not call her. She cannot be forced to take the stand against TM but there is nothing to stop them calling her as relates to MM. She was in the house at the time that a man was viciously murdered.
As for her being out of the limelight. I wonder if that is to stop any other slip-ups for the defence.
IMO the campaign may have originally been envisaged as a way for her to contact the children. She may have thought they may have set one up (I know many children under 13 who have FB accounts). It became clear very quickly from the Corbett family that the children did not have access to FB. However, IMO, something else happened once she started making those posts. She started to get attention. Many people who she could sing her woes to. From that moment it stopped being about contacting the children and more about people feeling sorry for Molly. Then it was all attention for Molly.