I posted here as it is the only place where we can maybe get some light shed on the issue. I believe this is a NW.js specific-issue, as even launching a third-party program using v0.12.2 does not allow the Steam Overlay to pop-up, but in v0.12.1, it does. I have tried all workarounds as described on as well. Through some troubleshooting and discovery, based on it appears as though Steam Overlay is completely non-functional in NW.js v0.12.2 on Mac OS X, but works fine in v0.12.1. Steam Overlay Doesn Work In Game Steam For Mac.
