Does WH recommend using his method for breath-holding underwater? (I don't know)
Hyperventilation causing deaths underwater is well-known and not specific to WHM
No, he explicitly says don't do that. Just one example:
https://www.youtube.com/watch?v=nzCaZQqAs9I&t=56s
Does WH recommend using his method for breath-holding underwater? (I don't know)
Hyperventilation causing deaths underwater is well-known and not specific to WHM