Re the cannon question: The case of Foxface does also seem to suggest that the declaration of death is not automatic, but rests on the subjective judgement of the Gamemakers. I'd also think that Gamemakers would be VERY leery of declaring a Tribute dead by shooting off the canon, only to have them wake up, as that would erode confidence in the Gamemakers, and likely lead to punishment for the Gamemaker. (I suspect that at least one Gamemaker was made a fall guy and punished for what happened in Haymitch's Games as well.) I can definitely see Plutarch using that as a cover reason for NOT shooting a cannon for Peeta; "Let's wait a little more! We DON'T want to make a mistake, President Snow wants this year's Games to be executed perfectly!" I wonder if this is another reason (in the books) that the bodies are taken away by hovercrafts, so that if a tribute does unexpectedly revive, it will be off-camera?
As for Gale and Anakin, I think they do have some similarities, but Gale never quite falls to the "dark side" the way Anakin does; the THG universe does not have any such concept, anyway. Also, Anakin falls largely because of his possessive love for Padme, who he tries to keep alive even at the cost of killing innocent people. I can't see Gale ever acting that way. While I think he's mistaken, he simply does not see the Capitol and its allies, even the children, as innocent, whereas I have a hard time believing Anakin did NOT know that the Jedi "younglings", or the children of the Sand People for that matter, were innocent (and Anakin killed the Sand People for personal vengeance, not any righteous cause).
While I think SC meant for Gale to be misguided, and not the right fit for Katniss, I do not see him as a villain. He is certainly motivated in part by vengeance, but he is not acting out of selfish ambition like Coin or Plutarch. Gale SEEMS to be thriving in his new MJ role as a D13 soldier, but I think what he saw in D12 scarred him as much as the Games scarred Katniss and Peeta. Also, he has only seen the negative side of the Capitol, since unlike Katniss, he hasn't actually spent any time there, or had quality time with people from there.
I also think that Gale would have been willing to give Katniss up for the greater good, which is quite different than Anakin. Indeed, you can argue that's exactly what he winds up doing. I also find it interesting how vague SC leaves his fate. His winding up in D2 could be interpreted one of two ways. One, that he went there as a conqueror, happily lording it over people he sees as morally inferior to himself, and ironically, turning into the same kind of oppressor that Thread was in D12. The other, more hopeful option, is that everything that happened DID get Gale to question his actions, and that he went to D2 to redeem himself, and actually wound up feeling solidarity with the people he had fought against earlier. Because if Gale wound up a "career soldier", he would actually have fit in quite well in D2. Though that's assuming D2 continued to be the primary source of Peacekeepers; I can see Paylor's administration breaking up the Peacekeeper system, and actually letting the people of the Districts recruit their own police forces.