Better than diceware? And sha512 related question
I don't really see the problem with just hashing the modified text. Even when the original text is public information, you've already shown that introducing $n$ independent changes gives you $\approx27n$ bits of entropy. Hence given an output hash, finding the modified text should be as difficult as exhaustive search if the hash function has pre-image resistance (and collision resistance, to ensure that the hashes of your modified text “look random“ conditioned on your original text).

Anyway, your line-by-line construction looks to me equivalent to some other hash function which combines multiple SHA512 invocations, so I can't imagine it'd be any more secure. Why don't you simply make a diff (e.g. insert a at position 12381, change position 812931 to f) and hash the result?