I've always felt bad for enchanters only because they will always have to mine, that is why I don't even bother with the class. (I hate mining) So, I don't think it would be too unreasonable to give enchanters a % of exp back when disenchant is used. Although I do think 25% would be a bit more fair.
As for thorns, I have only used one chestplate with thorns on it, and it dealt 10 damage back to a skeleton, I thought it was pretty useful. I personally like thorns, but I rarely use enchanted armor so I wouldn't really know.
(Sorry it had to come to tagging to get a response

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