The teacher guessed that some of the children <had cheated> <cheated> in the exam.

I was doing exercises in Advanced Grammar in Use, by Martin Hewings, and I got pretty surprised by the keys to exercise 5.2.

For example, he said that in 2 and 7 both the past perfect and the past simple are possible (where I thought the past simple is much, much better), but at the same time he said that, for example, in 5, 6 and 8 only the past perfect is possible (where I thought the past simple was also OK).

Can you please explain to me how it is that in 7 “The teacher guessed that some of the children had cheated / cheated in the exam” both are possible while in 8 “She said that she had made up / made up her mind who to vote for, and that I couldn’t persuade her to change” only the past perfect is possible?

(There's no context to any of those sentences)

The difference lies in the meaning of "cheated" and "made up her mind". Cheating isn't an instantaneous activity, like making up your mind. It might take time for you to make up your mind about something but when you actually make up your mind it's an instantaneous thing.

Which edition is this? The third edition?

I'm pretty sure that sentence 8 relates to note D about reporting speech, for which reason you're supposed to choose only the past perfect.

I don't have my copy of the book right here so I can't be sure. Could you post a screenshot of explanatory notes C and D along with the answer key so that I can confirm this?