Hello, I see you have discussed the issue of a chazan drinking. What about a regular mispalel if he started drinking before davening and hasn’t yet recited a bracha acrhona can he drink during davening ?
If a person is not well, and cannot daven without drinking in middle, then it would technically be permitted. However other than that, although it might not be halachically forbidden, ist is considered very disrespectful to that davening. Imagine a person who has a meeting with the king to plead for his life and needs. He gets an appointment and brings along his own coffee to sip during the meeting! We are a mere creation and should be awestruck when praying, ok we are not on that level, but at least we should how basic respect to the davening.