Their actions and omissions from the moment 'Victoria' was born can all contribute to the case against them as regards concealment, even though those actions and omissions may have had no legal value (vis a vis this charge, at least) until the fact of her death had occurred.
Respectfully, I think you are overthinking this. It doesn't matter whether the country at large knew by other means that a baby had been born. The only salient point is whether CM & MG met, or failed to meet, their obligations under the Act.
My opinion is that some of the lesser charges may get sidelined if/when the manslaughter charge demonstrates that it can go the distance - and if for any reason it doesn't, it may be that other charges, such as preventing a lawful burial get added in.
But another possibility imo is that 'free birthing' and generally failing to comply with the administrative obligations surrounding birth may be an increasing problem now that the UK has imported a certain amount of QAnon-style freeman-of-the-land type attitudes. If so, then the extreme vulnerability to which that kind of thing exposes children might be seen to make it worth making an example of CM & MG, imo.
But IANAL.
Freeman on the land movement - Wikipedia
Sovereign citizen movement - Wikipedia