Marriages are automatically conditioned on certain past events or lack thereof. If one's supposedly married partner turns out to be a psychopath, one has grounds for annulment.
If one's supposedly married partner lied about h past then one may or may not have grounds for annulment depending on the degree to which the marriage was based on the supposed truth of the particular part of the past of the partner.
For example if Jack marries Jill and then later finds our Jill once robbed a convenience store, I don't see a reason for annulment.
Some marriages such as those where divorce is not allowed cannot be conditioned on future events. Any such marriage where divorce is not allowed is invalid.
So there are some conditions on marriage allowed and some that are not.
What are valid conditions on decrees of legitimacy or succession in Game of Thrones/A Song of Ice and Fire?
Well one is the condition of legitimacy of the authority of the one decreeing. If tommen is proven to be illegitimate in authority or in birth, ramsay's legitimisation was invalid. But that's like the aforementioned psychopath scenario in the sense that it applies to all decrees of legitimacy.
What about something like the lying or robbing scenarios presented above?
For example, in the books,* Robb may have legitimised Jon.
My understanding is that
Robb talked it over with Cat who disagreed
He did that or planned to do it because he assumed Arya, Bran and Rickon were dead and that Sansa was a captive of the Lannisters
He would have named any of them his successors if the conditions were to his favour (well aside from being alive, there's also not being a captive of the Lannisters)
He would not have legitimised or planned to legitimise Jon if he could have named others successor
Could he have said something like
I Robb titles titles hereby legitimise my half-brother Jon Snow assuming so and so are dead as of [this date] unless I have children.
Part of that includes:
Can a ruler legitimise someone on the condition of the untimely death of another?
So if Arya, Bran or Rickon is alive or turns out to be alive, then there should be no problem for Cat, I think?
How about including Sansa?
I Robb titles titles hereby name my sister Sansa my heir upon my death unless I have children, she's married to Tyrion, (state more conditions). If she would not be my heir upon my death then I name Arya unless (...). If not Arya then Bran unless (...). Then Rickon unless (...) and if not Rickon I hereby legitimise my half brother without further conditions.
Part of that includes:
Can a ruler legitimise someone on the condition of the captivity status of another?
So he's still kind of disagreeing with Cat but at least there wouldn't be an unnecessary decree of legitimisation or inheritance.
I mean Robb wants to legitimise Jon because Robb thinks Robb's out of options. So why not just make a decree of legitimisation or inheritance assuming he's indeed out options?
Am I understanding anything incorrectly?
If he could have done it but didnt, I think this is an oversight. If he couldn't have, I would like to understand why.
Robb tells his mother that he plans to legitimize his brother Jon and name him heir should he and Jeyne not have a child. Catelyn is stunned by this proclamation, and pleads with him not to make the mistake the Targaryens made when King Aegon IV Targaryen legitimized all his bastards on his deathbed. The Blackfyre Pretenders plagued the Targaryens for five generations until Ser Barristan Selmy slew Maelys Blackfyre on the Stepstones. They both fear that Sansa will give the Imp a son and thus control of Winterfell and agree that must be prevented. She pleads that Robb consider naming his sister Arya as heir, but Robb insists that no one has seen Arya since his father's death and that his sister is likely also dead. Catelyn tells Robb that she cannot support his choice of Jon but Robb reminds her that he doesn't have to ask for her support, stating his reason as, "I'm the King."