Hey there,
#1: My biggest takeaway is that it is/seems pretty easy to do. I use breathing techniques myself (mainly for climbing hard routes) and love it. I also "make" my Kids do it but not regularly and mostly when there is some Kind of panik or unwell being.
#2: Again, no concerns - it's so straight Forward.
#3: Conscious Breathing and Visualization would benefit my child because it changes the Focus from the time he wants to achieve to doing the best he can.
Thanks a lot for sharing all this information, !