To start–as long as you’re not breaking the ToS, you can do pretty much whatever you want with your character’s story. This includes retconning. Retconning itself isn’t bad, and is a useful tool when a character needs a bit of tweaking. It’s not really bad unless you’re using it as a get-out-of-jail-free card to avoid reasonable consequences for your character, or doing it so often that it’s impossible for others to work with your story.
Multiclassing is also fine, and there’s many ways the classes in WoW can easily overlap. The classes themselves are more there as a toolkit to help develop a character, given that one OOC class can represent many different IC classes. When multiclassing, I think the two major points to consider are as follows:
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The OOC class visuals should fit the dominant IC toolkit.
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Opposing magical alignments cannot be multiclassed without a very good reason.
That is, it’s easier to RP the character in-game if their OOC class toolkit provides visuals that match with their IC toolkit, whether that be through transmog and weapons options, or the abilities themselves. Additionally, there are some things that it doesn’t make sense to multiclass together, such as demon hunter with priest or paladin(the magics are on polar opposite ends of the spectrum, and will thus be incredibly hard to manage together).
Mage and hunter fit together well, and neither has skillsets that contradict the other. Some hunters actually do use a bit of magic, given the existence of things like Arcane Shot.