That example in the interrogative does sound fine, meaning, as you said, "How dare he?"
What I said was that it was ambiguous. "How could he start his own business?" may, depending on intonation and the context in which it is uttered or written, mean either "How dare he start his own business?" or "How was he able to start his own business?"
Now, I have to tell you that all ESL books -- at least the ones I have used all these years since I was a student -- make a point of the fact that "was/were able to" has to be used when we need to convey the idea of past performance (past ability + actual performance).
In claiming that the "so (that)" clause in "He borrowed money from his parents, so he could start his own business
" may legitimately express consequence (= "He borrowed money from his parents, and, as a result of that, he was able to start his own business
"), I haven't claimed that actual performance is indicated. That is, I haven't claimed that (1a) implies that the referent of "he" did in fact start his own business.
Just because a person was able to do something doesn't mean he actually did it. And just as there is no contradiction in saying, "He was able to do it, but he didn't," there is no contradiction in saying, "He could do it (at that time), but he didn't" (even though we commonly express that idea, quite succinctly, with "He could have done it"). Neither "He was able to do it" nor "He could do it (at that time)" entails "He did it."
quote:The fire spread through the building quickly but everybody was able to escape. . . . (but not could escape)
Murphy's claim is that "could" is bad in that sentence, I strongly disagree. There is nothing ungrammatical or wrong with the sentence "The fire spread through the building quickly, but everybody could escape." Why is it that if I said, "The fire spread through the building," my interlocutor could legitimately respond with either "Could everybody escape?" or "Was everybody able to escape?"?