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Brain functions are important to maintain ventilatory drive. This is what drives the person to breathe. Besides a proper and full consciousness also assists in proper breathing functions.
In a patient with massive stroke
as both consciousness is impaired and ventilatory drive is reduces, this hinders breathing and efforts to breath; hence the mechanical/artificial ventilation. Swollen mouth/tongue has no significant functions to increase breathing activity. It can only cause obstruction which can be easily corrected by mouth gag's or head end elevation when there is an actual breathing effort.
I am sorry for your mother's suffering.
We all wish your mother recovers soon. Yet, based on what you say, the prognosis is guarded.