I mean .. why wouldnt Klara not know about her son having a tumor? Kei does (probably?) know about it too, but none of them probably knows about the serum (or does klara know?).
I interpreted the letter as Hera saying her final goodbye to her daughters? She did it herself with Aine at the fountain and Freya got a letter.
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This seemed very final to me and it looked like Jess made some kind of deal with her, containing Hera not interferring in her daughters lifes anymore and not pulling her in her own struggles. Maybe Hera is up for a final fight against whoever and doesnt intend to survive or Jess just thought its time to get finally rid of her evil shenigans towards Aine and partially Freya.
No way back, no redemption, Hera is just too broken and it seemed like she acted at least partially out of some misguided belief that her husband wanted Aine to become "strong".