Was Eru a Sadist?

[Child of the 7th Age]

I will respond only to the one issue that you raise, which is that of the istari and the limitations placed on them. This was not a matter of 'sadism'. Rather the istari were to teach and instruct Men so that the latter would be able to stand on their own feet and face whatever evil came at them.

Remember that the Fourth Age was to see the dominion of Men. What good would it have done if Gandalf and the others had come blazing in from the West and defeated Sauron with their own might? What would Men have learned? They would still be like children who are cared for by others.

It was only when things got very grim, after Gandalf fought the Balrog and was killed, that he was permitted to return with fewer restrictions placed on him. Even so, he was generally very careful to teach and persuade rather than directly confront.