Question

what do you think is the most difficult……这个用法对吗？

Answer 1

这个用法是对的。
What do you think is the most difficult？
do you think是插入语，what是疑问句的主语时就可以这样用的。
What is the most difficult?什么是最难的？
What do you think is the most difficult？你认为什么是最难的？
疑问词不是疑问句的主语时，要注意疑问句就再也不能用疑问句语序了，因为do you think已占用。
如：
What do you think they are doing?
疑问句：
What are they doing?

Answer 2

该句正确。可以把do you think看作插入语。

Answer 3

可以的，what 其实宾语从句的主语。