Because FCO goes to great lengths to explain the reasons for each move (unlike MCO), sometimes as much as a paragraph, there is no reason a relative beginner can't use it. Just realize that understanding the possible plans for each side is much more important then memorizing lines. And of course don't go crazy and try and learn the whole book. A handful of openings should be plenty.
Reviewing games should be done at every level. Ideal is going over them with a strong player. At the least play through the game and try to see what you could have done better before throwing it into an engine. Engines are great and will show you tactics you missed but they don't help you to think. If you get out a board and play out suggested lines though, they can be very useful.
Best book list I know of is Coach Heisman's:
http://danheisman.home.comcast.net/~danheisman/Events_Books/General_Book_Guide.htm
What is missing from Chess.com's Study Plan? I assume it should be supplemented, but with what?
Would a book such as Fundamental Chess Openings (or another opening book) be advisable for a "beginner" (i.e., someone under 1400)? Or should that come later?
The Study Plan (at least at the beginner level) does not mention anything about reviewing games or how to do so. Should this be done at <1400 and how?
What else is missing? There must be a section on this site about books to read and at what level. (I know there are many posts about books.) www.chessmasterschool.com gives a list that looks decent.