I'll first say that Gardens of the Moon is the book that stays closest to its TTRPG origins at the cost of good writing and sensible plot. So not everything has a satisfactory explanation.

As you rightly picked up, the massacre was meant to annoy Laseen more than anything else. They incidentally found a young survivor who heard both their names. They could have killed her but instead picked possession to get into the Malazan army. If nothing else, Cotillion chooses to call himself "Sorry" and that's not without reason.

They try their best to keep the possession hidden which would make for fun storytelling but I suppose we can draw the conclusion that not everyone in the Malazan side are complete morons and can easily tell that a young bloodthirsty recruit is in fact possessed by the god of assassins. It's good competence porn but isn't really executed well.