What is the exact mode of interaction. and how can it be established that such an interaction occurs?
Of course I can't know the exact mode of interaction between the soul an the brain. I am not the Creator.
What I do know is that the power of interaction must exist in order to enable the human freewill we all enjoy, just as I know the power of gravity must exist to stop us falling off this earth.
So the 'soul' is constrained while the brain of it's host matures - yes? If so, what are these neurological 'facilities' that gradually become amenable to 'souls'?
Or put more practically: his brain is developing.
Correct.
The ability to exert conscious control will gradually improve as our physical body develops.
However, since at age 3 you have concluded that he is making 'conscious choices' then, according to you, his 'soul' must be interacting with his brain so that by age 3 he has at least some of these 'facilities' you mention - so what are these and how do you know they are active? It seems to me that you are just asserting that this is so and that there is, therefore, the risk that you are wrongly ascribing to 'souls' what brains do naturally.
It is relatively easy to distinguish between natural instinctive reactions and consciously driven choices in most human beings.
One implication of your claim is that my 6 year old grandson, who also makes choices, must have a 'soul' - so how do I check that he has?
Can he witness to what he perceives in his own conscious perception?
Can he express conscious belief or unbelief in any context?
Can he consciously perceive meaning in sounds and words?
If so there is evidence of consciously driven control of his material brain, rather than instinctive programmed reactions to sensory data.