Opinions about whether Harry could actually destroy Azkaban?

In the sense of getting rid of the Dementors, probably. In TSPE he seems to think he might be able to do it, though with high probability of dying. In the last chapter, he was planning to take Fawkes with him, which is quite likely to have kept him alive. Note how Dumbledore says that he would have died after his duel with Grindelwald if not for Fawkes, that phoenixes seem to heal (give life, opposite of Dementors), and that a Patronus-nuke seems to be Cast from Vitality.