Using the taboo to track the unforgivables is a decent idea, but sadly flawed. As we've seen in the books, the taboo is not linked to active magic, but the sound itself. So anyone that says Harry Potter survived the Avada Kedavra will trigger the taboo. How long until the public knows that the taboo was moved and the DE avoid that particular incantation?
Mesmordere was a good choice as people didn't use the incantation in conversations and obliviators could be dispatched to preserve secrecy; critically important to the ministry as their very existence depends on it.
So the choice of taboo is not as stupid as you make it out. The best choice without doubt would have been Nagini to track down Tom's hideout, but now I don't know if there is a good choice. Everything but Mesmordere will generate far too many false alerts to be effective.
On another note, the Dursleys punishment is olay, but there's several issues that you didn't account for:
1. Shopping. You allowed them (well Petunia)to visit the grocer, but they'll need clothes, and Dudley school books at the very least; since you specifically mentioned the grocer, this should be mentioned as well.
2. Medical Care: It's indirectly said that they're allowed to receive it, but shouldn't there be proper procedure for them to follow if they needed to? Given all the specifics mentioned, it seems a rather glaring omission.
Anyway, this is a good story. Thanks for sharing.
Author's response
It still wouldn't be a problem. Since the Deez love to pay with that curse, and it's unlikely anyone else would, it still get's them caught. If someone else uses it, they're gonna have some fast talking. Unlike the killing curse, I cannot see a reasonable use for the torture curse, which is why I chose it.
My problem with Morsmordre is that they send it aloft just before they skedaddle. When the Aurors arrive, all they have left is bodies.
Nagini will be coming into play by June.
OK: Your specifics:
1. Shopping: For the time being,they won't need anything. the Dursleys have enough clothing to last them at least a year. Hell, I've got shoes that ate ten years old!
And Carolyn is at least a little pro-Dudley. She'll help him out there.
2. Medical Care: Yeah, I blew it there. Of course, We can always use the old wands. Since the muggle police will be made aware if the Dursley's punishment, I'd think any call for medical aid would be responded to, and as quickly evaluated.
Alorkin