Ok, so she spoke to Ken about wanting to harm Carla (missed that bit). She went to the flat with the intent to kill Carla, but changed her mind. Then she set down a lighted candle dangerously close to a lampshade. We don't really know if this was accidental or not. It could be a factor. Probably enough to arrest her? I'm not sure whether the fact that she has form for a similar offence can be taken into consideration at this stage (I thought this sort of thing was only admissable following a trial as it would unfairly prejudice it?) and she was eventually exonerated anyway, wasn't she?
I think the following scenario would have been more realistic:
Tracy blabs to Robert.
She comes clean to Carla, under pressure.
She is arrested for intent to cause gbh to Carla, or even murder. She admitted to taking the keys and entering the flat, intending to kill her, changing her mind over bashing her over the head, but instead leaving a lighted candle dangerously close to a lampshade. We don't know if the lampshade bit is accidental or not.
She is charged and tried. We need Ken to testify about Tracy's intention to harm Carla. He's a credible witness.
Tracy would try to claim that no way would she have done such a thing whilst her daughter was asleep in the flat. But there's enough evidence to prove that Tracy couldn't have known about Amy being there.
I suppose a successful prosecution would depend on whether a jury felt that Tracy was capable of committing such a crime. We know she is! I'm not sure how much other information about Tracy could be made available to the jury, though.
The deaths that resulted from the fire were an unfortunate consequence of her actions, but even if she were to be found not guilty of intended gbh against Carla, she would still be responsible for the other deaths.