I'm not at all inclined to find a person credible who argues that the answers to those questions are simple. I'm even less inclined to find a person credible who argues that the answers to those questions are unimportant.
Greg Beale's Handbook on the New Testament Use of the Old Testament: Exegesis and Interpretation surely is one of the most helpful books on the subject. It's a survey, so it doesn't address a number of the key issues in great detail, but it does offer a robust bibliography.
Justin Taylor posted a helpful overview of the book with links to related lectures and interviews. But Beale makes a crucial point that Taylor's overview doesn't deal with explicitly: The NT authors appear to have modeled their hermeneutical approach to the OT on the hermeneutical approach later OT authors applied to earlier OT writings. In other words, Beale argues that a careful analysis of how, say, the prophets quote the Pentateuch would reveal a similar hermeneutic to what the NT authors use when they quote the prophets.
If that proposition is true, then it would have profound implications for all sorts of contemporary debates, not the least of which is the tension among Covenant Theology, Dispensationalism, and their alternatives. And I suspect what might prove helpful is a partner volume to Carson & Beale's Commentary on the NT Use of the OT—a Commentary on the OLD TESTAMENT Use of the OLD TESTAMENT.