1: He counterattacks again until he can't kill anymore or he kills the wave...

This is really great for FW; tested it out like 3 days ago.

2: Lol I don't know

I really like RK, exciting to watch. I use him with relentless to get as many turns as possible along with what he comes with.  It is best to help out his speed, and over all attack power but the game is quite generous there. Set up right he will defeat a whole team or wave one by one. He is good on first turn but also on the return when enemies are weakened.

Based on my own experience, #2 results in an extra turn in addition to the extra turn for the kill. I had an arena match which featured an enemy Relickeeper in what must've been a relentless set. I killed the other 3, but he somehow survived just barely and got his turn. He then proceeded to, I kid you not, attack 8 consecutive times, not including the buffing between (which would be another 3 turns on top of those other 8), in which he attacked each member of my team twice which was enough to kill them and thus get another turn.

They  changed  Relentless  a  while  back.  Extra  turns  will  never  stack  anymore.