Even assuming it's possible to tell the difference between a few years old rotting corpse and a few years old rotting death doll (magic might help there), Dementors don't care about the dead, and a powerful enough wizard can easily sneak into Azkaban to replace a corpse. Finding a fake corpse in Bellatrix's cell would be a mystery, another conspiracy theory, not a sure signature of Voldemort. It might not convince anyone. Why settle for half-solutions?
Yes, I'm assuming that someone actively looking for signs of deception would eventually determine that "Bellatrix" was a deathdoll, especially if the wizard who created the doll intended them to. That seems pretty likely.
As for being convincing... well, unknown agents breaking Bellatrix out of jail with the vial isn't a sure sign of Voldemort, either, but it didn't have to be: Dumbledore inferred, entirely sensibly, that the expected disutility of Voldemort's return given the probability of it conditioned on someone breaking Bellatrix out of Azka...
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: