If I buy Course 1 in bulk for 4 students. I enroll 1 student. Then from your Enroll Student page, I purchase another product, Course 2 and add it to my Course 1 group. Now I enroll 2 more students. So will the 1st student be enrolled in Course 2?

Yes, student 1 will automatically be enrolled in Course2 when you purchase it and add it to the group Course1. All students enrolled in the group will be enrolled in the newly purchased course.