1/ Count Fosco's scheme works well. The Woman in White can drag for a while, mostly through Marian's narrative, but becomes fascinating when Sir Percival accepts the proposal and Count Fosco starts carrying out his elaborate plan (which, incidentally, might have inspired Fingersmith).
I presume the Count's a utilitarian whereas Marian and Laura apparently adhere to duty ethics, so if you allow to have your mind twisted a bit, then from the utilitarian's point of view, the scheme brings about the best outcome- everyone benefits in some way from it, nobody gets hurt, Anne's already sick and Laura doesn't care about wealth anyway. The Count also drops a hint for Marian, which he of course doesn't have to do.
2/ Frankly speaking I think, without the hint, Marian probably wouldn't think of checking the asylum, at least for another while. She's no match for the intelligent, cunning Count. If the women in The Moonstone- Rachel and Rosanna and, in some way, Lucy, obstruct the plot, the women here either get fooled and unknowingly help the villains or stay helplessly passive.
Once Walter comes back and takes over as the narrator, we can see the difference right away. He's smarter, more careful, active, decisive.
3/ The conspiracy or the result of it is interesting. The discovery however is quite disappointing, like the bit about laudanum in The Moonstone. Again Wilkie Collins needs the naivete and gullibility of a woman, and it's Mrs Clements.
And yet, in spite of everything, I have to keep reading. For the secret.