Kevin,
1. Office files like word and excel work just fine, check out an check in. the check in with file for office files appears after the file is closed. .AI files do not. further when we try to upload the file instead of check in we get an error, can not overwrite this file is checked out.
2. yes, all mac users experience the same.
3. our share point o365 team site via web access does show the file as checked out.
4. we downloaded the program from 0365 web site i believe, but i am not 100% sure.