I think Seth and his disease are just there by accident, not by plan. Callista and her children most of the years lived on their own outside of Karlsson's reach. Seth could have easily died and the Karlsson's would not have noticed. They intervened just when the Game had started ca 3 months after Karlsson's death and at that time Seth was almost dead, a hopeless case unless you spend millions of dollars and use best equipment and personell. Seth was (un)lucky to still be alive to participate in the game.
It seems that the world in KG is much more dangerous than our own world today (unless you live in a war zone.) So even Kane could have died by accident, in a fight with the wrong people, having a car accident, getting cancer, etc. Longtime plans based on only a few individuals are too uncertain for important matters.
If Seth's disease is caused by a genetical defect which is inherited from and so is present in one or more members of the big clans like the deKock or Karlsson, then the genetical defect is not new information and should be known to Veronica from previous cases. So Seth is more like another Guinea Pig to find a possible cure for a rare disease (which might be not so rare among the Karlssons and/or DeKocks.)
Ch6+7 flashbacks already established it more or less that it's not a coincidence that the concerns you stated (mc or Seth dying) didn't come to pass.
Mistress Callista had access to a helicopter & Otto. (also a car iirc, even tho mc & olivia state to the karlssons that they only learned to drive through a car repair shop job they had - i.e. no personal cars)
If Seth really was in danger, & Callista wanted to save him, she had more than enough avenues.
"Longtime plans based on only a few individuals are too uncertain for important matters." - yet the Gambit is such a plan. Which is why I believe nothing about the 3 siblings was left to chance. Either by Callista alone, or with help from Alexander and/or the DeKocks.
"I think Seth and his disease are just there by accident, not by plan." - well, you can't plan a genetic disease after all.
If you consider Seth's parentage as uncertain, it be theorized that Seth was found & carefully preserved upto a certain condition (the brink of death) just for his condition.
We can see Veronica experimenting on similar cases of variants of the disease(Veronica science work in ch5). But breakthroughs only happened
after Seth was acquired by her. Even the dialogue by the island4 scientists mention Seth specifically (counterpart - speculation).
So we can maybe consider that Seth's variant of the disease may have some significance.
Note: There's not much Seth stuff in the story yet to accurately theorise about him. Even the Seth's pregnancy thing was used by me to formulate a Olivia theory instead of anything related to Seth.
Even all I said is related to other hints (like Callista preparing the 3 for the Gambit, etc) instead of direct Seth hints.
I only related Seth to Noah cause of the biblical connection, & it's always unreliable to use naming to guess these stuff.
I already asked once about Seth, he's one of the most open ended characters so far, but I'm guessing we'll get more into him in the following chapters, since the island4 transfer feels like a (new) beginning for his larger arc. Maybe even a flashback (after 2 consequetive Olivia flashback chapters, it's be good to change it up a bit rn)
