No. іе -> ё, іі -> ї, иі -> й. оу -> ў. They are just shortened, to write less.

Well, ў for once is not a shorthand for оу (e.g. слаўны, ўзяў). But it's beside the point, all diacritics are used to "write less".

> But it's beside the point, all diacritics are used to "write less".

That may be the reason for misunderstanding. In my language (Slovak), diacritic is not used to "write less". It is used to express different sounds, i.e. s = с, š = ш, c = ц, č = ч.

Sure, but just as you demonstrate you have fewer distinct characters when you use diacritic than when you don't. And arguing whether Е and Ё are same character with diacritics, or just different letters that happen to have diacritics by the virtue of canonizing in alphabet is really splitting hairs. They serve same purpose, in the same circumstances, with the same visual arrangement. Quacks like a duck etc.