I agree with all this, but I think make her perish by her image and make her totally useless and having no control on what remains to her of life is a stronger statement than making her "useful".

Graendal is destroyed, because she'd been crushed down to her image, the real woman behind all those masks is gone. In the "useful pet" scenario, her brain's still there, it's just her image which is destroyed.

Aran'gar's "theme" was that of a parasite. He stuck to Aginor, she stuck to Delana and Sheriam, and held on to Egwene, and her whole storyline parasited Mesaana's. Then she went to find a new host to parasite with Graendal. Graendal just did some pest control as (I'm convinced) Mesaana and Demandred have done to get rid of Aran'gar (it wasn't Aran'gar who killed the two women tied to her, of all things. We saw Aran'gar kill people in her way, and she was very careful not to leave traces). The first move was to have BA (Moria) convince the Hall the Cleansing was a Forsaken weapon, to bait the Rebels to the BT. The idea was to capture sisters no doubt and to use the disappearances to delay as much as possible any post-Cleansing developement (Demandred had seen mixed gender circle back in play, he needed to try to minimize the spread of that), but a side benefit was also to frighten Aran'gar away. Didn't work, she held as the Hall was dithering and dithering on the embassy. Semirhage thus leaked the information about Cabriana, Demandred sent one of the Asha'man to target the two friends (or another male channeler went, we'll see what he really has in AMOL), again a side benefit was to keep AS and Asha'man away (didn't work - Moria lead the first blow but lost control immediately as Lelaine jumped on the opening and took control of the whole Asha'man debate) and another was to get Aran'gar captured if she just wouldn't take a hint and vacate Mesaana's backyard. That still didn't work. The final nail in the coffin was, as a last resort, given to a Black Sister (Nacelle, IRRC): a weave to detect saidin weaves (the very thing Moghedien was so proud she held off from the girls...), as it seemed the AS just wouldn't make the connection between Halima and Anayia/Karen on their own. Romanda was just a bit faster.

Her end wasn't stellar, but it was fitting.