Apart from Ernie's theory and Dumbledore's theory I don't recall any other explicit explanations suggested for Harry's survival within the original works, and there are unlikely to be any because, out of universe, any such alternative explanation would have undermined the impact when the truth was finally revealed.
Ernie's theory wasn't a problem in this respect because from the viewpoint of the reader it was obviously wrong. Snape echoes a very similar idea at the beginning of Half-Blood Prince when speaking to Bellatrix, though he goes on to dismiss it based on his first-hand (if biased) experience of Harry's skills.
Dumbledore, of course, acts as a stand-in for the author on at least two occasions in explaining what actually happened once the story reaches the point where it is appropriate for him to do so.
It is possible that later works (e.g., Pottermore) offered alternative theories as presented by other characters, once this would no longer directly spoil the original plot, but if taken seriously that would still undermine the mood of the original works, in which Harry's survival was a great mystery to which nobody but Dumbledore had the answers. (Well, maybe Xenophilius Lovegood, who might perhaps have published some wacky theory involving one or more Crumple-Horned Snorkacks. That wouldn't undermine the story quite as much - except that one would be left to wonder why Luna never mentioned it.)
As a side note, your question is also explored in the second part of chapter 81 of the fan-fiction novel Harry Potter and the Methods of Rationality - search for the word "nervous" to jump to the relevant section, and note that it includes one very minor spoiler.