5.1.1 Can students outside the U.S. access the at-home exams?
Yes, though please note the exact timetables for the exams. This year, exams for a given subject will be given on the same day at the same time (simultaneously).
5.1.2 Is the Chinese/other countries firewall/connectivity an issue?
If the students can access My AP or AP Classroom, they should have no issue accessing the exam on test day. But there are unknown local settings that can cause issues on exam day, so this is not a surefire indicator.