I went—not overseas, but by public transport—to Corpus to hear Tamsyn Muir speak.
The bad news: Alecto is not finished. It will hopefully come out "soon" and will likely be fast tracked with few ARCs when it's finished.
The news you may take differently depending on your preferences: It is not being split.
The good news: It is not all written in Ye Olde Alecto speak (it sounds like Harrow's POV will be fairly major, but there will apparently be several narrators).
A slightly random selection of things I frantically scribbled down:
The protagonists of TLT would make an "absolutely shit" D&D party ("Palamedes and Camilla would be fine")
We could have had horse plinko and begone thot, but for the anti-meme ministrations of her editor. She would love an edition that puts all of the memes back in.
On Catholic imagery and lesbianism: "you ain't seen nothing yet"
"Harrow is now a believer without a church"
She said that while John and Alecto's relationship is not meant to be a 1:1 analogue to Humbert Humbert and Lolita, there is the idea of a man fashioning (something he thinks is) a girl into a perfect partner (the question of whether that is a sexual partner apparently may be relevant to ATN)
She does not have a favourite House and would just be a regular person in the world of TLT (though she would last about 0.5 seconds)
The tension between the Houses' ostensible gender equality and the misogyny that still persists is apparently also relevant to ATN. "John has set out to make a society on values he holds dear and cherishes and in some ways he has done really well... And in some way he has fucked it up beyond comprehension" (Maybe not an exact quote. My auditory processing is questionable.)
The backstory in NTN was planned right from the beginning
Lyctors "are not truly human any more. They've crystallised themselves" and "They have lost themselves and the only thing they've been able to hold on to is what other people make of them". She said she would have liked to make the Lyctors more alien but had to balance that with them being relatable narratively.
She is dying to read TLT fanfiction once she finishes the series.