Locker codes in order are: -32, 16, 4, 12 -7, 54, 21, 48 They don't change, at least on fresh files.

Other than that just memorize item locations and practice the puzzles. There's very little variation in items you have to find on fresh files.

I don't know if replaying the game on the same file will make the playthrough any different.

Lastly, you should always play with your x-ray device out, there are no downsides to using it, and some objects can only be seen by using it