Am I reading the rally mechanic for her correctly? If you replace a d6 roll with one rally die, you aren't getting to roll that d6 again, it is replaced with a 4? Hence the reason she can't gain rally dice that same turn?

If that's the case, she has the potential to get a lot of extra critical hits...

I happened to pull her. Can confirm deadliness. She also comes with the Mercy sword, which gives her even more options for manipulating an opponent's attack rolls. The green rally symbol means that unless you get extremely unlucky you are going to be collecting rally dice uses quickly. If she's hit while on her stop click, don't forget to use a rally die to give yourself at least a 50/50 chance of avoiding the attack.

Scarab is the perfect sword for her. Rally 4, replace dice in attack roll with fours, double fours are crits, and increase damage and super senses rolls by +1 for each 4 in the attack roll. Makes an already dangerous piece just straight up brutal.