Who decides spellings ?

[b]Lexicographers[/b] People who's job it is to record and compile dictionaries in the language of their nation.

If they're smart, they'll research where the word comes from and explain the foundation of it.
Of course. If they're not. You get Americanized versions which are not only spelled 'differently' but can also have alternate meanings altogether.

Eye due.

It's a collective thing, no one in particular.