"If your civil service works correctly then this should not happen - and would anyone else like it, to have their Government make a policy that retroactively applied to them with criminal implications?"
This is a good point - the discussion with the police on 'hypotheticals' which even the police saw through, suggests people were likely to be getting carried away with the enormity of what they'd been made aware of & were looking for ways to try and 'nail' the allegations. There's the Q of political expediency too - very fortuitous for some certainly.
And I think this is the key - this process was developed alongside the knowledge of serious accusations & that in itself informed the path this followed for development.
However that came about, the inability of anyone senior who had responsibility on this to see their own failings is still seriously concerning.