Yes, you effectively ignore the threat hexes.
And, yes, he effectively always slams back.
However, it's probably easier to not think of it that way.
Mind Like Water allows a player to use their opponents own momentum against them.
Also not that Mind Like Water has been errata'd in the Season 2 rule book:
Mind Like Water works as written with the single change that the player uses the same stat value as his opponent to roll against as well as using the same number of dice. So, if his opponent rolls 4 dice and needs 3+, then that’s what the player with Mind Like Water needs to roll as well. Note that Coaching Dice can be added to either side’s roll and are included after the calculation of how many dice are to be used. This is the only way in which the opposing players could roll different numbers of dice.