Primogeniture help

[Dannyalcatraz]

I need as the story is best served by a strong legal argument so we can see their choices around it.

To get a legal argument, you need a legal system. And as pointed out, those vary from culture to culture.

 

[John Dallman]

We have a case where someone had forfitted their titles and their son took them up. If they later had a child and the son died without heir, would that child be in line to inherit?

Well there are several possible sequences of events, but they all have questions on them.

First, was it defined when A's titles were surrendered to his son, B, if subsequent children of A (call them C, D and E) could inherit if B had no children? The king would have had to be involved in A giving up his titles, and if he were wise, he'd have made sure that the issue was settled in advance to avoid a potential civil war.

If that was not settled, one interpretation is that C is (necessarily) younger than B, so could inherit if B died childless. The other interpretation is that C was fathered by someone with no right to the titles. and thus could not inherit them. And that's where the backers of C, and the backers of Q, A's younger brother, both have an arguable case, and the result depends on legal and cultural details about your setting that we don't know.

 

[Ruin Explorer]

That's the point - I am looking to define how the culture does it. Not based on "the correct" way in the real world, but in such a way that it sounds genuine to a RL lawyer who is one of my players.

Here's your problem - historically this kind of thing was never, at any point so well-defined and reliable that a modern lawyer would give it the time of day.

It was consistently vague, ill-defined, contradictory and subject to whim and politics. If you look at real-world examples of succession you will see this. Particularly when regencies and the like get involved. As Benjamin illustrated upthread, England's one was probably as sane as it got (that I'm aware of - I suspect some less-well-recorded culture somewhere did better - but we may never know) and it wasn't great.

 

[QuentinGeorge]

There are a few examples from history that are at least close to what you are saying:

John Bailliol abdicated the throne of Scotland to be eventually replaced with the Stewart dynasty, yet Edward Baillliol was able to succeed to the throne with English support. (though he was already born at the time of the first Bailliol's abdication)

A better example was Phillip V of Spain, who abdicated in favour of his son Louis, and then only Louis' death reigned again, and then after he died was succeeded by his OTHER son, Ferdinand. Though again, both sons were born prior to abdication.

You probably won't find an example of the exact thing you are asking as abdications tended to occur either shortly before death (natural or otherwise) or at least towards the latter part of a monarch's life when they already had plenty of heirs.