Release, the old version is just there for those who want to convert from old to release.
No, you can either do in-game with less precision due to lack of decimals, or do config file directly with better precision.
Multiplier is useful for finding the best multiplier for one specific scope, then use this multiplier when selecting sensitivity.
Some might be missing if their in-game value is set to the default. If you change everything in-game they should appear.