If you enter 3 as the value, +1 will always be added making it 4 (Quick maths). I'm not sure if this is intentional or if there's anything you can do about it, but I just wanted to make it clear that it exists.

Impact seems to add +0.5.
Not the case, the values are just weird. I put 2,5 on diamond longsword hoping it would bump it up to 3. Ended up with 1.6

The configuration value is regarded as an adder. Default value for hit_at_once(updated name is max_strikes), impact, and armor_negation are 1, 0.5, and 0.0.

Above the 5.1.0 version, It uses Datapack system to define a weapon. It will overwrite the value that you set in configuration file.