I just checked the game now as it has been a bit back and forth exactly how it handles the sensitivity. It's now back to how it originally worked.
Config file 1 is just the value displayed in-game, if you change this in the config file it will not actually change the sensitivity. Decimals are supported and recommended, as it will be more accurate when displayed in-game.
Technically yes, but the drawback is that the in-game sensitivity will not display what you actually have. If you do both it's easier to get an overview in-game.
This value is not used unless you have "Universal Sensitivity for All Scopes" enabled. And all it does is setting all the scope sensitivities to the same value.