>>"Operation not Allowed: You cannot nest sequences with different editing timebases."<<
All three issues are caused by the same thing. When you see that error message, it means that some of the clips have a different frame rate, which is why when you try to paste them into a non-matching sequence the audio goes out fo sync. You'll need to identify which clips frame rates don't match and convert them so that ALL frame rates match the Sequence settings.
Why nest at all? Copy and paste all the clips from one sequence to another.
You will have stuttering issues with the footage that doens't have the same frame rate....FYI. FCP does not do a good job of mixing framerates in a way that looks right. Nesting wouldn't change that...even if it worked.
Nesting is evil...need to avoid.