Have you seen it? I's not very useful sidokar. His grade book series however is very good but maybe easier for an adult. As a child I needed lots of help to understand it. There maybe better online courses for adults.
Yes of course, otherwise i would not have mentionned it. It is a summary book, ie it is not a full course with examples, and so on. The value of it is 1. It gives you the exact content of what one needs to know and 2. It is a good handbook, where it is easy to find quick answers and reminders when you forget something.
But if one needs more explanations then additional resources are necessary. Most of the grade 5 is about syntax, notations, ... i have many theory books in different langages and some of them are not that much more detailled. Some topics are pretty basic so really you dont need anything else, some others may need some more materials.
Like any exams, to pass the abrsm you have to 1. Fully master what is in scope 2. Do plenty of past exercices to make sure you are acustomed to the type of questions and have assimilated the content well enough to answer correctly. Until you do the exercices you actually dont know if you have really understood the concepts. Sometime i think i do but when i actually practice, i realize i misunderstood.