I could be wrong but I'm pretty sure that to be the legal heir to the baronetcy, a son would need to be legitimate - ie born in wedlock. So Penelope Clay would need to get Mr Elliot's ring on her finger before she gives birth in order for the child (assuming it was male)to inherit the title.

I should think the same would apply to the property of Kellynch Estate, which is subject to an entail, as implied by the line in chapter 1: "There was only a small part of his estate that Sir Walter could dispose of;".
This leads me to wonder whether the entail confined inheritance of the property to males only, as seems likely. What would happen to the estate if William Elliott were to die without a male heir. Would it pass to a male in a different branch of the family (perhaps a Dalrymple) or could it be inherited by Elizabeth or by all three sisters in default of heirs male? I don't think we have enough information to decide this, and I certainly don't have the legal know-how, but it is fun to speculate about.