The admin account on AEM is like the windows administrator or the Unix root account: if you have that, you can do everything. You should provide a secure password and only use it when necessary. In many projects it is typically stored with the operation team because they require it for some operations and also analyzing issues.
specifically to your questions: You can write an observation listener to check for changes to /home/users/a/admin and send out emails when the login-token is changing.
for oak-reindexing: no, that's not possible, because this is secured by ACLs, and for admin no ACLs are checked.
HTH,
Jörg