To answer your questions:
1) yes, he needs to prepare all documents before apply online.
2) no need to wait. He could send paper to Thai embassy after filled online form.
3) yes, make sure that he calls and asks Doctor first and make an appointment.
4) some takes 1 week, some takes 2 weeks. 20+ days should be okay.
5) you can a 100% get Covid insurance online from Thai Insurance Association :
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