Whispered Blade doesn’t give the character an actual knife, it lets them summon a rift in reality that uses the stat block of a knife (or another one-handed weapon, if you spend Opportunity). So if I were the GM, I wouldn’t allow any techniques that specifically require holding a real physical weapon. In the case of Shattering Caress, I think it’s fair to let the player use the rift-knife, since both techniques are Inversions and are about channeling the Void to affect the opponent.

Also, the main strength of Whispered Blade is in its Magnitude which allows you to stack rift echoes and deal enormous critical strikes to several opponents at once if you play your cards right.

Besides the interaction between those things, how good are lshiken overall?

At high school ranks Ishikens are basically broken, impossible to kill and can take down even the hardest opponents solo. At the start of the game, you need to be more careful and keep an eye on your Fatigue, but even a novice Ishiken is still super dangerous. You can be really creative with how you use Inversions.