Wow, it is late so if I miss something, sorry...
It is illegal at any level to target a player with the crown of the helmet, helmet to helmet is not facemask to helmet, or if they happen to bump heads, we are talking full on cheap shot launching blow to the opponents head, linemen hit helmet to helmet every play...
It is a personal foul to target any defenseless player or players obviously out of the play... Ex.. OLineman is trotting up field during long run and defender decides to look him up, or vice versa well behind the play, PF..
There are a lot of alternatives and play situations would definitely help, some of these are even hard with descriptions because there is a lot of subjectivity and judgement in these calls so hard to say one way or another.
Afaik, there is no requirement in 6 man to have any specific # of players or one side or the other as there is in 11 man where there must be 4 on each side of the ball at the kick.
KO should be from the 30 unless relocated by penalty.
Receiving team can step in front of the restraining line and initiate contact with the kicking team, but the kicking team cannot block R until ball has travelled 15, if R initiates the block it should not be flagged, but just because R stepped across the line isn't license for K to block.