1. Can be an issue; only on the purest of the pure collectible Colts for originality, would it make a difference IMO.
2. Yes my 1st gens are brass, but it's unclear to me if they all were brass. 2nd and 3rd gens are not brass.
3. Yes, 2nds appear to be carbon steel. 3rds must be stainless steel, they won't take bluing.
When the spring is compressed and it tries to form a figure S instead of staying straight, it's worn on the sides unevenly from rubbing on the inside of the housing. Notice some coils will be flattened. Time to change for me, but if they're still work properly, just keep them clean and oiled to mitigate further wear.