!) Yes, you can use those interchangeably. 2) We don't really say 'the hardest conditions'- it just sounds odd.

The first two sentences are very similar and both are correct. "He has a lot of debts." is a flat statement. "He has so many debts." adds an emotional component to the statement. It expresses a feeling that there is something bad about the number of debts he has. In the last two sentences, "harshest" and "hardest" are equivalent. But the last sentence is incorrect without the word "for."

Last sentence doesn't make sense. Use "We're ready FOR the hardest conditions."