I remember DA himself saying somewhere that some of the most wonderful projects were never on his project list; they just got done! And one of the reasons behind the necessary relaxed focus was that his mind was free to focus on them since all the open loops were out of his mind onto his lists.
GTD is a theme, a human idea, not a hard and fast computer algorithm, to get things done. So I suppose there is no such thing as pure GTD. Take the theme, and tune it, refine it, fill in the details, etc as suitable for you. The central ideas are to capture everything, clarify your commitments, and review them periodically (various levels of commitments with different suitable periods) in order to decide what to do in the moment. Whether you make a daily list out of your committed next actions (once-a-day review of actions), or check it everytime when you want to choose the next action depends upon your personality, your type of work, how you enjoy working, which way you feel less bothered about the system itself and get working, how fast your inputs change, and several other factors. Your intuition will usually give the answer.
Same thing can be said about working contextwise. The idea of context is not to restrict you from taking actions; it's just eliminating reviewing actions that you cannot do right now anyway. And the review all context lists usually will let you decide when to be in which context so as to move appropriate projects ahead. If you feel you can make some significant progress on a project sitting in your office for four hours, definitely go head; your office is the context. Your next actions are not the only choices that you have; they are triggers that let you kick start towards your goals. Once in the momentum, you may want to continue working without bothering entering and removing actions into the system. It's only when you want to stop (lunch?, 6:00pm?, meeting?) that you may want to enter into an action list what you would do next given the next chance to work on the same project.
Choosing the right actions or right projects to work on requires a feel of what is right. This comes with reviews of the system at various levels with various frequencies. These frequencies usually depend upon several factors such as written above; usually the entire inventory is better reviewed at least weekly. Choose frequencies appropriately.