Yes. The official questions, at least in the books increase in difficulty as you move forward. Even in online question bank you can filter by difficulty. So doesn’t matter. But more on that below.
Good and important question. Plan mostly suggests you to go through videos (on GMATClub and GMATNinja YouTube channels mostly) for concept building. These videos while excellent in explanations, might not necessarily seem comprehensive and building confidence as they lack topic wise/chapter wise approach. It is expected that you supplement with GMATClub sets. However, if one still needs detailed concepts just doing sets won’t help in my opinion. The approach is fine for verbal and DI to a large extent but Quant is where you could feel inadequate. The videos in general assume that you are already decent with basic stuff and not always feel like they have a definite start and end. They deal with one question type for each concept and move along with increased difficulty. Compare that with some prep company that deals chapter wise and covers all concepts, question types. But be cautious, for you might lose too much time trying to gain undesired mastery in Quant topics. Law of diminishing returns. So the 25 questions sets suggested in 13 week plan might not necessarily be to your liking if, say, you get frustrated seeing a question that you did not have strong basics to begin with. Hence better find something (Khan’s academy/GMATClub quant book, Prep company, etc) if you are someone who needs detailed concepts and their practice to perform better.
For Verbal the videos are adequate if you can supplement by grinding LSAT sets and apply the approach outlined in videos on consistent basis. DI in extension too is mostly Verbal and Quant combined. You might however want to practice as many different types of charts for Graphical Interpretation as I felt that knowing the nature of graph is important before going to exam.
I personally had already completed all basics before (having taken official exam few times) and felt that someone completely new to GMAT Prep without solid basics might find this 13 week plan somewhat frustrating.
BUT, the plan is an excellent compilation of best resources and cuts all unwanted stuff so that people need not get stuck in this prep for several months and years. It makes sure that if you can do what’s in the plan, supplement with resources as you feel necessary, and practice hard as suggested you can get this exam over with in 15 weeks - With thorough practice, consistent review and analysis of what you’re doing wrong, and concept building being the essential ingredients.