While having a surrogate might be legal, it is more complicated than you think, because to be eligible to be in the line of succession the baby has to be born by PWBW herself. This is why there actually have to be the queens doctors in attendance as they act in lieu of the witnesses of the past when a royal bith was a more or less public spectacle to make sure girls weren't swapped for boys etc.It's not possible that the RF didn't know the bump was fake, they have eyes everywhere, but they played along. This isn't complicit as H&M are adults,, it's legal and their responsibility.
A surrogate born Merchie would not be eligible for a place in succession.
I'd also wager them slipping a surrogate born child into the line of succession would be treason. Real treason. I am not a lawyer and I haven't read up on this particular aspect of the surrogacy theory but my gut feeling says this would be treason and I'd bet a tenner on it anytime.