Hi siximig, welcome to the Gibbon community and thanks for your kind words 
Sounds like you’ve made some great progress searching in the forums and documentation. I’ll answer your questions as best as I can, in the future it can be helpful to post these as separate topics, because it will help people who are looking for similar answers.
Looks like there have been a couple other people looking to use Gibbon for a music school, these posts may be of use: https://ask.gibbonedu.org/discussion/2265/adapting-gibbon-for-use-in-a-music-school/p1 and https://ask.gibbonedu.org/discussion/1065/setup-for-music-school-without-fixed-school-year-or-semester/p1
- Classes are instances of a course, so for example Piano lv1might be the course, and if it was taught to three different groups of students, you would want three classes to track and timetable them. For small schools though, each course usually has one class.
- Lessons are the materials taught on a particular day for that timetabled class. Lesson planning can be useful if its handy to have the class materials available online, however it's not required. To begin with, just getting your timetable and classes setup would be the first hurdle.
- Your friend & her secretary would first enrol the student in the school year through Student Enrolment, and then into their classes through Timetable Admin. For a music school the year groups and form groups may not be as useful (unless you have different year-based progression for students). One way people have bypassed this is to setup one year group and one form group for the whole school. This will depend on the school's structure.
- Google calendar integration doesn't turn Google into a timetable, but allow external school-wide and personal calendars to be displayed within Gibbon. It's largely there as a visual aid, to see all daily lessons and events in one place. There's no connection between the Google calendar events and the classes/timetable in Gibbon.
Hope this helps!