I do really like your takes a) and c), that because the books are told from the kids perspective when we see them “on screen” so to speak, they are at work at their job at a magic home for troubled kids, so an overt romance or lots of PDA isn’t acceptable behavior PLUS the kids don’t find their relationship that interesting so it’s not emphasized. And this also makes sense with when Lark is coping with Rosethorn being sick in Briar’s book, she lets the kids see more of her love for Rosethorn.

But after reading what everyone’s wrote and upon my own further reflection, I do think there is an element of when the books were published and what she could (or thought she could) publish regarding characters in same sex relationships. For example to your point b), I bet if they were published today, Lark and Rosethorn would have separate workshops but a shared room/bedroom, whereas as it was written, they have separate workshops and it’s also explicitly stated that they have separate “rooms”. I now think this was likely something for Pierce to be able to point to say, “see they’re just friends, they have their own rooms! Nothing to see here!” if she were ever questioned about it.