Tips on learning this very specific subjects that are not in the scrum guide.

We can test and learn right here if you like. Criticize and discuss the following statement:

"A good Product Backlog is a schedule of work used to forecast when a customer will be fed with a release"

Have you gone through the SM learning path as it covers your problem areas? 

Scrum.org_SM_LearningPath\Managing Products with Agility\Product Backlog Management

Scrum.org_SM_LearningPath\Managing Products with Agility\Forecasting and Release Planning

Although it is the PO & PAL learning paths that have a 'Stakeholders and Customers' section, if you search this site for blogs pertaining to stakeholders, you will find enough information to help you pass the exam. 

I would suggest a class if you really want to learn the subject and how to apply it. The Scrum.org training classes don't just teach Scrum, they are highly activity based hands on places to learn while doing so that you can apply the results in the real world. If you only focus on the test, you don't bring that experience, I suggest you look at improving your knowledge and understanding at the same time.

Other options are look at the Prepare for the Assessment section here: https://www.scrum.org/professional-scrum-master-i-certification

Sample tests don't teach you much more than right and wrong answers, but classes and educational resources teach you how to think and apply.

Also based on your test results please make sure that you know the Scrum Guide very well. I am seeing incorrect answers that are almost word-for-word out of the Scrum Guide.

I hope this is helpful. 

One thing that helped me was to look at the problem space and try to understand why it was important.  Then I would rephrase the problem and search for material on that. To this day I continue to do that to learn more.  For example

• Product Backlog Management

The Scrum Guide says that the Scrum Master is responsible to the Product Owner in

Helping find techniques for effective Product Goal definition and Product Backlog management;

In my mind there are a few key words: "Helping", "Goal definition", "Backlog management"

Knowing that the Product Backlog is a single source of information about all things that need to be done in order to improve/maintain the product, I focused on understanding how to capture information of that nature.  After I started to understand how that information is captured, I focused on learning how the empirical nature of Scrum could be used to order items so that the most valuable are nearer the top. Part of that is learning the difference between order and prioritize.  There could be a feature that is the highest priority to deliver but other things might be ordered higher because of quick wins or dependencies. 

After learning that I started exploring how goals can be useful for more than just setting a target.  Goals can also be used as a data point in the ordering activity. They can be used as an element of risk assessment.  There are many uses.  Learning them helped me understand more of how they can be effective in managing the Product Backlog.

I hope you get the idea.  Don't focus on a single 3-4 word statement. Focus on the words, the intent of the words, the intent of the exercise.  Learn the meaning and use as it applies to the Scrum framework and to a company's ability to inspect and adapt as new information is learned. 

The PSM exams do not focus on rote learnings. They want you to apply your understanding to the questions.