Sweet scent does not increase shiny chance, it only makes you get a horde.
Shiny chance: 1/4096 in X/Y and onwards
Shiny charm increases chance by 300%, so
For answers 1 and 3, finding a horde naturally is a 5% chance if you step in grass if the area can have hordes. Just take the numbers from 2 and 4 to get the answer.
Hopefully this is correct, or at least close to the answer. I might have messed up with something because I am not the best at math, but if any math experts notice its wrong, I will change or hide this
Either way, its a really small chance.