pronunciation is like that of English and any other language, it's called an accent and there are always multiple forms of accents. You might speak English but not have an American accent (hypothetically speaking, since i don't know you) instead you might have a British accent or something similar to that or not ever that. Like different parts of Europe, Asia, Australia, Africa, etc - English has multiple dialects and accents. Which, like i said earlier, is the same with all languages. There's the accented pronunciation to everything.
When talking about someone else that one is not familiar with, one always uses the informal form (sie, er)?
There's only one way of talking about someone.
It's only when talking to someone that there's the choice between formal and informal.
otherwise "The apple is for her" would have been accepted here also right?
The sentence uses Sie (you) and not sie (her; them).