5Are these sentences correct?
Active: The staff have left the office.
Passive: The office has been left.
Strictly speaking this is correct, but most unusual.The active sentence is far more natural.
John said that the lake would probably freeze tonight, because it was much colder than last night.
Fine. If you are reporting John's statement earlier in the day, you could also say, "John said that it will probably freeze tonight, because it is much colder than last night". If you are reporting it the next day, or after that, 'tonight' becomes 'that night', and 'last night' becomes 'the previous night/the night before'.
"I have left some books on your table", said John. "I think you will find them useful. You can keep them as long as you need them but I would like them back when you have finished with them." => John said to me that he had left some books on my table. He thought I would find them useful. I could keep them as long as I needed them but he would like them back when I had finished with them."
Fine. Once again, if you are reporting a recent utterance, back-shifting is not essential.