I've never heard of an injunction granted to prevent reporting of a criminal offence, never mind a serious criminal offence - and even if it was given, the outrage over it would probably result (as we saw with Trafigura) in an MP reading the report out in the Commons anyway.
No idea what has happened in this case, though I would genuinely be amazed if she has reported that and nothing has been done (at a minimum one would think once that allegation has been made and they've seen that evidence that he would have been arrested).
As for the media personally I think if its true they've sat on this for months its probably for the same reason they sit on loads of other information; its potential use to them (I mean this sort of coercive pressure is probably how a lot of the ITK info from the dressing room gets to the papers in the first place). As Leveson showed they've got extensive form for obtaining information by means most would consider foul (or illegal) and this would fit in, I think.
Finally (if it is true they sat on this) then the largely negative coverage of Rashford in the right-wing press really needs to be looked at again; imagine slating someone for taking a principled stand on feeding kids when you've also been keeping this quiet.