ya i'm wondering how come they stopped showing yechan, my guess is the family wasn't comfortable with people knowing he has a speech impediment, but i don't see any problem with that cause i find it cute since my little bro has the same problem.
it can be both. for children, it's safer for them to use a fist because it protects their fingers or sometimes their heel if they're very small. adults can use the side of their hand or even things like their head!