a) OK, but I think for the very first question, it would be better to say "What do Grant and Flash do?" rather than "they".
b) Fine. You can also say "Which one of them is a bad builder?" or "Who is the bad builder (of the two)?"
e) Fine, but you should also expect the correct response "No. Grant is a better builder than Flash."
f) Fine, although of course many things have probably happened to Flash once. Only one is mentioned in the story. You could perhaps say "What does the story tell us once happened to Flash?"
As happens sometimes with your comprehension questions, they are a mixture of questions where the question and answer contain words which are found in, and can be taken directly from, the text, and questions where the students have to show that they really understand the wording of the question first.
a, b and f are all very easy to understand and to copy the answer.
c, d and e are better (in my opinion) because, for example, c) and d) use "How ...?" If the student doesn't understand that single word, they can't answer the question without just guessing.
Personally, I would make e) more challenging by changing the question to "Is Grant a worse builder than Flash?" The word "worse" does not appear in the text so you are testing whether your students understand the connection between "better" and "worse".