"I was planning a nigh-complete rewrite for the book - less than 50% previously published sentences, say. Would it be a problem if the ideas, but not sentences, all exist elsewhere?"

You might need to remove some posts from the net if you decide to use them as is in the book, but if it is all modified, there shouldn't be a problem AFAIK.

Ideas can't be copyrighted, and you wouldn't be the first person to turn blog material into dead tree.