Is the international pool more competitive when applying to boarding school?
Yes
I would say so is the FA pool, and the non-hooked pool. Triple whammy if you are all 3.
Depends upon the specific school and depends upon whether or not the applicant is seeking financial aid.
Many prep boarding schools cater to the international market–especially to those who are full-pay applicants.
Depends on the country and the school.
There are some countries where there are very few applicants: Myanmar, Laos, Sub-saharan Africa, smaller latin american countries. For these countries you would have a geographic advantage. Countries like China have an overabundance of applicants, so its much harder. Whether you can pay in full also matters a lot in decisions.