Michael B. Rappaport has posted Reconciling the Unitary Executive and the Opinions Clause on Notice & Comment. Here is the first paragraph:
One of the strongest arguments against the unitary executive interpretation of the Constitution is based on the Opinions Clause. If the President has constitutional authority to direct executive branch officials, why was a clause that authorized him to “require the Opinion, in writing, of the principal Officer in each of the executive Departments” needed? After all, even without the Opinions Clause, the President could simply direct a principal officer to provide him with a written opinion. Under the unitary executive view, then, the Opinions Clause appears to be redundant.
Highly recommended.