It is expected that music teachers in general, organists in particular, can automatically train a choir based on varying amounts of experience of singing in choirs (or not) themselves. This is where I hope I can help, much of which I'm sure will be confirming what you already know but also clarifying and filling in some gaps that you might find useful.
Much of my time as a singing
teacher and pupil is spent undoing bad habits and releasing tensions built up since infancy. As
a new born baby we could all scream for hours at the top of our lungs without getting tired -
The body in complete alignment, joints and muscles free of unnecessary tensions, breathing purely
a matter of filling the lungs. Over the years we pile on emotional, psychological and physical
tensions, our posture often becomes distorted causing shoulders, necks, jaws and tongues to
tighten, we try to compensate for these problems and build up a further layer of bad habits
reinforcing them the whole time.
It is also different in that we carry our instruments with us the whole time. Any tiredness or
illness affects its quality and no two instruments are the same. Everyones voice is unique and
no two people experience the same sensations.
There are certain basics that have to be right but generally learning to sing is a journey of discovery - experimenting, recognising the best sensations and most efficient use of resources. It is the choir directors' job to develop a good ear for healthy sounds and reinforce them with consistency, good habits will then emerge.
Some of the best teachers of singing and top vocal coaches are not singers but have a finely tuned ear for the voice whereas a singer can fall into the trap of imposing their own perception of sensations on their students. The sound and means of production can then become manufactured.
With a good ear and basic idea of how the voice works, it is not necessary to be a singer to make a real difference in someones singing. The main danger is that certain stock phrases have been passed down through the generations, many of which contradict one another or have no grounding in fact - Sing through a hole in your forehead is one that personally comes to mind. Although in the upper register there are some sensations or secondary vibrations transmitted through the skull, this is an impossibility and can be misleading. It might be a quick fix for some but will result in tensions for others. I'm not in favour of quick fixes, magic buttons or miracle working, but good habits being formed for life.
Not all music educators are in favour of training the child voice before the vocal folds have completely adjusted to the physical changes related to puberty, but I see no reason why good habits for healthy singing can't be set up as early as possible. This does not mean teaching 'vocal production' whatever that means, but being aware of what can be expected at each stage of development, nurturing them and ironing out any problems that occur as early as possible. There is no harm in reinforcing correct breathing, experimenting with the voice, encouraging expressive singing of songs with a suitable range and tessitura and working on clear diction.
Developmental Stages Ideas for developing good singing habits Structuring a choir practice