RNRB Meaning of "Closely inherited"

I have a scenario whereby “D” cared or a minor children “C” full time due to the minor’s biological parent was incapable of looking after the child for a number of reasons. This was a permanent arrangement whereby C lived with D as a family unit. However, the LA were not involved with the arrangement. D was never named as a foster parent and no applications were made to the court in line with S5 Children’s Act.

D & C both refer to each other as “Mum” and “daughter.” ( this provides background and perspective only)
D was the point of contact with the school and the school were aware of the arrangement.
C is now an adult (mid 20’s). D has approached me to understand her IHT position and estate planning.
D has no other children.
Jurisdiction is England.

D has an estate exceeding the £1million and so the RNRB would beneficial if it can be claimed. However, I am not convinced the conditions of 8K IHT 1984 have been met. subsection (6) does not appear to have been met due to the facts above.

Due to lack of awareness of the need to formalise this arrangement via the LA, it appears the conditions have not been met. D explained that it was never necessary to do this. If this were a minor child in such an arrangement today, I am sure the school and LA would forced the issue and arrangements would have been encouraged to apply via the court for parental responsibility.

I wonder if it possible to retrospectively formalise the arrangement, but as C is now an adult, I am not sure that this is possible.

My question is, does anyone have some helpful comments or recommendations? or have you attempted to claim the RNRB with similar conditions? Any routes to consider would be helpful.