That raises so many questions i do not know where to begin. First of all, it seems that, at this time (it is fairly in the beginning of the book), Siona is still not completely invisible to prescience but just, like, developing the trait."I have seen the report on Siona," Moneo said.
Leto's smile widened. (...) From his own traverse through a similar evolution, Moneo knew with precision the delicate nature of Siona's present fortunes.
"Have I not created her, Moneo?" Leto asked. "Have I not controlled the conditions of her ancestry and her upbringing?"
"She is my only daughter, my only child, Lord."
"In a way, she reminds me of Harq al-Ada," Leto said. "There doesn't appear to be much of Ghani in her, although that has to be there. Perhaps she harks back to our ancestors in the Sisterhood's breeding program."
"Why do you say that, Lord?"
Leto reflected. Was there need for Moneo to know this peculiar thing about his daughter? Siona could fade from the prescient view at times. The Golden Path remained, but Siona faded. Yet ...she was not prescient. She was a unique phenomenon... and if she survived... Leto decided he would not cloud Moneo's efficiency with unnecessary information.
Also, it pretty much spells out that no, prescience-invisibility was not part of the plan, it was not imagined beforehand nor wanted. It happened and Leto was very happy about it, but that seems to be it.
So: how much can Siona be the main point of the GP as some (myself included, for sure) have come to presume?
Was it, like, Leto guessed there should be something against prescience but was not sure what? And then comes Siona and finito? Might be, but i am not completely sure.
On the other hand, He states with all possible clarity: "have I not created her?" but i dunno, it could be said equally of Moneo and He was not very excited about Moneo...
The passage about He seeing Harq-al-Ada and not Ghanima in Siona puzzles me, since Siona will be in the future taken as "archetypal Atreides", and that would mean that she has not that much Atreides in her... But it also could mean that He does not see the "Atreides part" of Siona, that a part of her is completely invisible even if she's got the military (Sardaukar, belligerant, self-destructive) side of the family too.
{Maybe there's more in the context there, the whole preaching-against military thing... (or not so much against but sideways) I quoted what seemed relevant, but the whole chapter seems connected in ways i do not understand completely...}
So: is Siona the key of the Golden Path? Or no?