It is usually more difficult for international students to get into US colleges and universities. This is because many schools have quotas for how many international students they will accept, so you're competing for a very limited number of spots with other people from all over the world. Unfortunately some colleges will also discriminate based on an international students' ability to pay. You should check whether a schools is need-blind for international students or not before you apply if you know you'll need financial assistance.
Typically, the only material difference in admissions requirements for international students is that they must demonstrate English language proficiency by taking the TOEFL. However, for international students, extracurriculars and SAT scores usually matter more than grades in the admissions process. It's tough for colleges in the US to know the context of your high school grades, so standardized tests are given more weight. Extracurriculars are also important because schools are most interested in international students who will contribute something interesting to the community in terms of cultural diversity and a global perspective (super cliche, but true).
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As for your second and third questions, we actually do have an answer that partially addresses these issues, which you can read here. Specifically addressing the first part of your second question, international students applying to top colleges from US high schools may have it slightly easier than those who are applying from outside the states. While they still have to compete with other international students for a small number of spots, these students' high school grades will be considered more seriously in the admissions process, so there's less pressure on standardized test scores. This is because colleges can compare their transcripts more directly to those of domestic students. In a US high school, you'll also have a more direct view of the admissions process, meaning better guidance on how to navigate any issues that might come up along the way.
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The spike concept outlined in Allen's article is relevant to all students who plan on applying to highly competitive colleges. I think it's actually even more important for international students to have certain specialized talents or achievements that make them stand out from the crowd. As I said in my answers to the first three questions, admissions are more competitive for international applicants at these schools. That means you need to have something really special going on to get your foot in the door.
There's really no difference in the advice for international students except that your spike should involve something that's easily recognized an a major achievement by American schools. Starting your own charitable organization, winning an international competition, or creating a new invention are all potential spiky ideas. It depends completely on your interests and goals.
I know that the spike advice can be confusing because you're always told that being well-rounded is so important. This isn't an either/or situation, however. Both are important, it's just that the spike is MORE important. For example, you're better off starting your own profitable company but having a below-average GPA compared to other applicants than you would be if you didn't have the spike and you got absolutely perfect high school grades and test scores. A spike is more important than well-roundedness because it actually makes you unique. A spike won't save you from a super low GPA or test score, but if you're in the average range for scores and GPA, it can take you from a 50/50 chance of admission to pretty much a sure thing.
To answer your final question, yes, you can be recruited as an international student athlete by colleges in the US. The main difference is that it's your responsibility to reach out to the coaches at these colleges and show them your credentials. They won't go overseas to see you compete, so you might send a highlight reel and, if the schools is interested, attend a camp or showcase for potential student athletes. For more information, you can check out this site.