conditional, second, third, combined

[Tukanja]

Please someone tell me which of these sentences are correct.

1. We would have highly appreciated if this review was signed.
2. We would have highly appreciataed it if this review was signed.
3. We would have highly appreciated if this review had been signed.
4. We would have highly appreciated it if this review had been signed.
5. We would have highly appreciated if someone had signed this review.
6. We would have highly appreciated it if someone had signed this review.

Is the word "it" really needed in the sentences? (What about ellipsis)

Thanks

[Josef]

4 and 6 are possible. "It" is necessary.

You could also say:
We would have appreciated it highly if this review had been signed.
We would have appreciated it highly if someone had signed this review.

[Tukanja]

You could also say:
We would have appreciated it highly if this review had been signed.

Thank you for the additional way of saying the sentence.
I like to see "highly" after "it" and not "appreciated it if" kept so closely one to another.
"Appreciated it if this" is hard to say for me.

In addition you've led me to say the sentence this way

We'd have appreciated it highly had this review been signed.