A nice summary, Peter. The important practical point is that it should be a very conscious decision about which fiduciary is responsible for investments. Then, work through the plan terms to make sure that that fiduciary is is named by following the correct procedures for identifying the various fiduciaries and their roles. The worst thing that can happen is that someone becomes a fiduciary without knowing that they bear that responsibility, by default, by mistake, or by ignorance.
One hopes that plan terms, or trust terms, provide express authority with respect to appointment of fiduciaries. Sometimes the authority to appoint is implicit. A recent question in the message boards about use of custom documents is relevant here. Custom documents, drafted with regard for fiduciary and other ERISA concerns, rather than simply tax compliance, will probably have more understandable and accommodating provisions with respect to management of investments and other fiduciary duties. The IRS is not looking out for those aspects that apply to plan operation and liabilities.
Also remember that most of the time a fiduciary that appoints another fiduciary for a specified purpose, such as identifying and managing investments, still has some responsibility to monitor the activities of that fiduciary.