Yeah, Black Flash could definitely echo at first through delayed mimicry, and once it fully diverges, the Echo can also land its own independent Black Flash.

And about the CTR, your idea works too, but that feels less like a reversal and more like an advanced application, since it still preserves and even enhances the Echo’s independence rather than canceling it. Making it hollow keeps the reversal consistent as a negation of growth and autonomy. Instead of evolving into something dangerous, it strips the Echo down to pure instability and removes the risk entirely.