Does accidentally inhaling smoke (whether cigarette smoke, incense, or cooking fumes) invalidate the fast, and what is the extent to which this is considered an invalidator of the fast?

If you unintentionally inhale the smell of smoke or incense, and it reaches your throat, your fast remains valid and is not invalidated by it. The same applies to food vapor.

A person afflicted by obsessive doubts (waswas) may adopt the lenient opinion. Jurists do not consider mere smelling as breaking the fast, but rather the intentional inhalation of the substance itself into the throat.

You should not intentionally inhale such things. We advise you to constantly supplicate and humble yourself, disregard these obsessive thoughts, and consult a trusted psychiatrist.