If the plot required Graendal to have an angreal later on the simplest way would have been for her to have worn her ring angreal when Rand balefired her palace. In fact, it would be highly out of character for a paranoid Forsaken NOT to wear her angreal with another Forsaken present in her palace.

Actually, there is no reason for Graendal to EVER take off her angreal, as it is a small ring, and she is always wearing rings.

So the most logical conclusion would have been for her to be wearing her angreal and therefore she would not have lost it in the first place.

So the traded angreal from Mesaana was not a necessity of the plot.

Regarding Graendal being mindtrapped, her scene ended with her being taken away by Shaidar Haran for punishment. The same thing happened to Moghedien, and she was not killed, merely mindtrapped, despite failing to a far greater degree than Graendal has.

Besides, it doesn't make sense to kill off your powerful channelers this late in the game, if you could mindtrap them instead and direct them as weapons in the Last Battle.