I know exactly what you mean with making her call on turning Denji into a human weapon, and i defnitely believe this can go both ways. I feel there's a 45% that Yoru's move actually works and Denji becomes a weapon, and 55% that Denji isn't phased by Yoru due to some specification of him being the Chainsaw Devi that makes him immune to Yoru or maybe it's something that conflicts with Asa's personal feelings.
Basically,
Yoru's plan might not work either because the Chainsaw Devil is resistant to Yoru.
or
Yoru's plan doesn't work because Asa's personal feelings for Denji are interfering.
However since this is Fujimoto, we know it could go in any direction and it's been a long time since I've had this sensation for a manga series. The last time I've felt this way about a series was from Tokyo Ghoul/ Tokyo Ghoul :re from Sui Ishida.