Yes, it can, depending on the word. Prefixes, however, almost never change the root word. The most common changes of a root when adding suffixes are the removal of a final E that follows a consonant (e.g. diving), or the change of a Y to an I (e.g. silliness).
I don’t know the context of this but: we must choose ourselves because if we are trying to be someone else our whole life then we won’t get very far and people won’t like us for ourselves.
I think the answer is "each other" for all of them?
He was a spokesperson of his generation because his books mostly spoke about people who experienced suffering due to the gilded age system of values.