Lord Fatty could have overturned the entail if he had wanted to, that's what a lot of Cora and Violets scheming was about in Series 1.
So my guess is that M&M will adopt Baby S, Pickle will bugger off to be all revolutionary like, M&M will find that they cannot have a child.
In remorse, Lord fatty will agree to overturn the entail so that Baby S can inherit after M&M die.