Are those 3 subjects important for the selective universities in Mongolia?
Is there any way you can include Chinese (or World History)?
Indeed, in the US, Business and IT are not “core” subjects but elective. They’re not considered as academically serious as others. In addition, if you’re interested in Public Policy (and International studies) you’d need a “Humanities/Social Science” subject. It’d also offer better justification and correlate better to studying Public Policy (a major for which your odds are better than Business or IT/CS).
Can you keep Chinese (or World History) or substitute it without endangering your prospects in Mongolia (or other countries)?
Your academic profile is excellent and your GPA close to a 4.0. (Note: a UK B is equivalent to a US A-.) Your extracurriculars are also very good and coordinate well with Public Policy/International Studies.
Yes, when applying to the US, you always have to keep in mind that you might not get into any college.
Are you looking into other countries?
For instance, UToronto has a full ride merit scholarship for internationals - worth looking into.