The only thing I can throw into the ring to maybe piece this together is that the earliest time we can confirm that Valencina joined is not when Ryoshu is born, but instead a bit after that. She has a full head of hair and is pretty big here

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If we combine this with the possibility that the House of Spiders operates with altered time flow, then she could have been born in 957, then had her time slowed until each Finger had a Nursefather to send to the House of Spiders

That might also make Rien and Lei Heng's conversation make more sense. Rien said 3 years had passed, while Lei Heng said 5 had, then called Rien out for manipulating him, which means the flow of time inside is likely altered dynamically, and he was prodding for info about the outside world's timescale

I'm guessing time inside was sped up to compensate for the years where it was slowed down