Question about one decision in chapter 3 (spoilers)

[Bilateralrope]

How can leaving Alvin with Shani instead of Triss even look like a good idea ?

By the point you have to make that decision, Salamandra have already taken him twice. Once in chapter 1, and again in chapter 3. It's clear they are going to try again. Triss is a sorcerer, Shani is only a human.

From the mage glossary entry:

Only rare individuals have the potential to become mages and many of those with this potential are doomed to madness. Unless the individual in question — known as a Source — learns to control their power quickly, he or she may end up a half-insane, slobbering oracle

Sure, Triss is part of a female only group of sorcerers, but they should at least have some plans to deal with males who have magic.
Shani has no magic training. So she can't help prevent Alvin going insane from his magic. But she could bring the plague home from the hospital.

Shani lives in a rented room, with an old woman below* who doesn't seem to like anyone. Triss owns her own house in a much more expensive part of town. So Triss is in a better position to care for Alvin.

*Unless you kill that old lady, then I don't know who inherits that house, or if they will let Shani keep renting it.

But the very worst thing against Shani: You did leave Alvin in her care in chapter 1. She quickly game him to the local witch.

Am I missing something ?

Every other big decision, and all the small ones I can think of, had enough ambiguity to them that I can see why someone would chose either option. But this one is very one sided.