My own (thankfully) limited experience and reading suggest that there is often an interplay between different professions in these factitious disorder cases. Sometimes an over-excited doctor will run away with the idea, sometimes he may be getting (incorrect) data from other individuals mixed up in the case. As this is not an illness in the ordinary sense, he can't just do his own examination and say, yes, this patient has a pulse rate of X or blue spots on their tongue, and therefore it's a case of MSBP. It is partly an exlusion diagnosis ("I can't find anything else"), partly based on social judgment of the parents- and in the latter area, he may well listen to the other experts, such as SWs.
He may have to rely on information provided by SWs as to how these people interact with the child, and they in their turn will have to rely on him for the medical judgment. In this respect, it is far less a medical diagnosis, and more of a social one.
The reason our own case did not to to court was twofold: a mental health nurse made the judgment that we weren't really that kind of family, and we were fortunate enough already to have an appointment with another specialist, so did not have to rely on the doubting doctor for a referral.
The second time trouble brewed, it was a headteacher who believed we were causing dd's problems- he was firmly quelled by sensible SWs who refused to act. I am very grateful for those SWs.
What we can learn from those old abuse cases, is that sometimes SWs and doctors ended up in a kind of folie a deux (or trois or quatre) where they were egging each other on to believe more and more impossible things, and that it was in this interface that sometimes professional judgment got blurred. The only thing I can see that would help would be for each part to try to be as informed as possible about the limitations of the expertise of the others- and of course, this is where a good judge could also do wonders.
In factitious disorder cases, it would help enormously if doctors were more honest about their limitations, and if a very clear distinction was made between parents who could reasonably be believed to have harmed their children and parents who were simply voicing a different opinion to the doctor. Putting a mother who is actively poisoning her child into the same category as an over-anxious mother who worries that her child may be seriously ill, is just ludicrous.