Are there any other (in-universe) theories about how Harry survived Voldemort's attack?

Question

Ernest Macmillan from Hufflepuff had said in Harry Potter and The Chamber of Secrets, chapter 11

"No one knows how he survived that attack by You-Know-Who. I mean to say, he was only a baby when it happened. He should have been blasted into smithereens. Only a really powerful Dark wizard could have survived a curse like that."

He dropped his voice until it was barely more than a whisper, and said, "That's probably why You-Know-Who wanted to kill him in the first place. Didn't want another Dark Lord competing with him. I wonder what other powers Potter's been hiding?"

Are there any other theories like this that are mentioned by any other character in the book, or other works of J.K. Rowling on Pottermore?

Answer 1

I suppose Ive to answer my own question now.

Found this passage In Harry Potter and the Half Blood Prince, Chapter 2

I should remind you that when Potter first arrived at Hogwarts there were still many stories circulating about him, rumors that he himself was a great Dark wizard, which was how he had survived the Dark Lord’s attack. Indeed, many of the Dark Lord’s old followers thought Potter might be a standard around which we could all rally once more. I was curious, I admit it, and not at all inclined to murder him the moment he set foot in the castle.

The conversation between Snape and Bellatrix Lestrange (Questioning by Bellatrix rather than a conversation).

Same Book, Chapter seven.

Slughorn talking to the slug club in the train.

“Of course,” said Slughorn, watching Harry closely, “there have been rumors for years. . . . I remember when — well — after that terrible night — Lily — James — and you survived — and the word was that you must have powers beyond the ordinary —”

Looks like Ernie's theory wasn't unfounded. There were talks about this theory before Harry joined Hogwarts School of Witchcraft and Wizardry.

Answer 2

I had thought it to be canon that Harry's life was saved due to the Sacrifical Protection Counter-Charm:

Sacrificial protection is an ancient, powerful, and long-lasting counter-charm. It is endowed when one person (whom we will call "the victim" for purposes of this article) ultimately sacrifices their own life willingly and out of deep, pure love to save the life of one or more people (to whom this article will refer as "beneficiaries"). Some people regard sacrifical protection as "the ultimate protection".

Dumbledore even explains it in the very first book. From the wiki I linked above:

Your mother died to save you. If there is one thing Voldemort cannot understand, it is love. He didn't realise that love as powerful as your mother's for you leaves its own mark. Not a scar, no visible sign... to have been loved so deeply, even though the person who loved us is gone, will give us some protection forever."

He elaborates on it again in The Order of the Phoenix:

But I knew too where Voldemort was weak. And so I made my decision. You would be protected by an ancient magic of which he knows, which he despises, and which he has always, therefore, underestimated — to his cost. I am speaking, of course, of the fact that your mother died to save you. She gave you a lingering protection he never expected, a protection that flows in your veins to this day.

Answer 3

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.