Regarding Sandaria accompanying Aivah and Merlin to the cave, it may have a simple explanation. Remember that the SSK is a bit divided on the meaning of using Spanish and where Adam and Eve's came from before the creation of Safehold:
HFQ Chapter 2 wrote: “I don’t know that for certain, because he never explaned his reasoning in the portions of the journal we can read. Until I encountered that ‘Español’ of his, it had never occurred to me another language might even exist! And however reasonable it may’ve seemed to him at the time, his decision to use it means even the Sisterhood’s members are divided on at least a few points.”
“Oh?” Merlin tilted his head, and Aivah smiled more than a little tartly.
“Some of us — myself included — have interpreted the passage in which he recorded his decision to begin using ‘Español’ to suggest that it came from some time or place which predated the Creation. Combined with a few other puzzling references, one could almost read that as saying all of the Adams and Eves were . . . somewhere else before Safehold was called into existence.”
Her dark eyes were suddenly very intent, boring into him like twin blades, but she went on calmly, almost tranquilly.
“Even those of us who read it that way are divided about where that ‘somewhere else’ might have been. Most of us interpret it as evidence that not even an Archangel could create a soul — that God Himself must be the sole Creator in that sense — and that all those Adams and Eves were with Him while the Archangels prepared the world in which they would live. But a fair number of us think he might just as well have meant the Adams and Eves lived and breathed on an entirely different world and that God and the Archangels brought them here from that other world, rather than first giving them life on the Day of Creation. It’s a substantial distinction, and one I’ve often thought we could have found the answer to if he’d written the ‘Español’ portions in something we could read. Or,” she added, raising both eyebrows, “that the Sisterhood could read, at any rate.”
“I might be able to do a little something about that,” he acknowledged slowly. “I can’t promise. And you’d have to trust me with the journal — or a true copy of it, at any rate.”
It may just be that Sandaria belongs to the other "faction" in SSK and they want to show her the truth and it kind of fits in my opinion.
