''As soon as they had opened the statue they found a message inside'' though I am not sure if it is possible to use past perfect with as soon as
"Inside" has the same meaning, but you are asked to translate "v ní" here. "Inside" is "uvnitř". The Golden rule says you should translate as closely as possible.
"As soon as" is accepted.
No I am saying 'had' should be accepted
Ok, but sentences are always evaluated as a whole. I will add "had opened", but not "inside".
Be prepared for the past perfect to be missing in many places. You are welcome to report them though.
Thanks that you take your time to improve this course, I think many people don't include past perfect anyway but just to be grammaticaly correct. I will try to report anything I can.