In fact, κι is the correct. Euphonics demand this, instead of και εμείς.

εμάς is an accusative and genitive pronoun, εμείς is the nominative. This sentence is purely talking about the subject -- i.e. Buddhism and us -- and lacks the predicate (verb and object) which the accusative would appear in (i.e. the εμάς variant of which you speak). The sentence contains no genitive clauses (i.e. something "of us") so again εμάς is not correct. The only possible answer (to my mind, though I'm happy to be corrected) is "Ο Βουδισμός και εμείς."

"Us" translates to both "εμείς" and "εμάς".

United States of America=USA/US (capitalized).