> You can't fill your lungs as full
> neglects using diaphragm, which is the primary muscle for breathing
Which are bad because...? These two have nothing to do with whether it's a relaxed activity or not, by the way.
> it creates tension in chest, neck, and shoulders
How?
> singing resources (singing well -- and also playing any woodwind or brass instrument -- requires natural relaxed breathing)
Singing or playing a wind instrument are not relaxed activities. In both cases the breathing muscles are being used to force air through a resonator to produce sound. Search images of trumpetists or classical singers and tell me that's how a relaxed person looks like to you.
> As a real world example, you may be able to notice in yourself how when you become nervous or scared your body becomes tense and you tend to chest breathe. Focussing on belly breathing in a case like this can actually calm your emotions as well as your body, dissipate the fear or nervousness, help you gain control over your fear.
If you have time to think about your breathing then whatever is scaring you can't be so bad.
Focusing on a specific thing, no matter what it is, will distract you long enough to calm you down a bit. Haven't you heard that thing about counting to ten when you get angry? Same thing.
Look, I'm not arguing that using the diaphragm is not the most efficient in terms of energy expended / volume of air exchanged. I'm a trained athlete, so if I didn't know this I would just pass out within seconds when I work out. I'm saying that once you're exchanging sufficient air to cover the aerobic budget of whatever it is you're doing, exchanging more will accomplish nothing. If breathing with your chest is enough to cover it, for example, while sitting (as it clearly is, as no one has ever suffocated from doing it), then for breathing with the diaphragm to be more efficient it has to expend less energy per inhalation. So far you have done nothing to show that this is the case.