They do try to represent the various main types of problems that are going to be there - such as on critical reasoning you have all of the main question types: strengthen, assumption, weaken, most useful to know, flawed reasoning, method of reasoning, inference, paradox, bold-faced reasoning, and Plan questions of various types.
Now, are those questions all at the rates that you will see them on test day? Are they of the same difficulty level that you will see on test day? Not necessarily. It all depends on things like difficulty levels. But you will not get a whole new type of question that is just not in the OG at all.
On the Quant, you will find that the major topics are all included, but the questions may not cover the full range, such as using permutations to calculate a probability.
You say,
Therefore, If I study and completely understand all the problems in the book, will that be sufficient to get a decent score?
Completely understand the problems as you are doing them as in get them all correct as you work through them - certainly you could get a decent score! If you mean understand them as you read the answer explanations, well that depends on whether you can recognize and apply what you have learned when you are dealing with an unfamiliar question. That is why it can be good to use other resources, even before the official guide.
If you go through, for example, the Veritas Quant books and have a good idea of what to do with various problem types and you understand how to identify those types then you can go to the Official Guide then you are actually practicing the "recognition and application" that is at the heart of being a great test taker, instead of using the OG to try to learn. Using the OG to learn is a waste of that great resource. Better to learn from a book that provides strategy (rather than trying to develop strategies for yourself) and then use the OG to practice. 
