If the Head of a major power is so weak that it can't be criticised then maybe the power needs replacing
I don't think anyone is suggesting that the RF or the institution of monarchy should be beyond criticism.
What's being called into question here the basis for that criticism - its motivation and its authenticity.
H&M's salvos at the RF are driven by self interest, not out of any desire for reform or change in the UK being a republic - if they felt that way the first thing they would have done is renounce their titles voluntarily. To quote H when asked the question of retaining or relinquishing his titles "what difference would that make?".
So the motivation here is hardly worthy of kudos and the authenticity of the allegations has unraveled significantly post Oprah with the pair of them unable stick to their own brief on the matter when it comes to the details (or lack of them) and frankly quite astonishing revelations as detailed beautifully by the OP around the issue of race in H's interviews and "gene" dominance.
If they want a dialogue about the institution of monarchy and to be taken seriously then they need to up their game (and put their skin in it).
Instead I think rather than weaken the position of KC and W they have strengthened it - because a lot of people simply respond to their antics as thank goodness W&C are next in line and not H&M.