- Students can change their own email address
Isn't this the default behaviour in Moodle? Role Authenticated user has capability moodle/user:editownprofile.
- Students cannot change their own email address to another address that is already used
This is the default behaviour for user accounts using manual authentication. If you're using a different authentication plugin please provide details.
- Customer advisors can combine two accounts, merging one student account into the other // this keeps progress in their courses (does not reset)
- When merging, the old email address is logged - During merge, the student advisor can see EVERY change that will happen before clicking GO Other system:
the enrollments system finds out about this and merges sales orders to the new student account (maybe using the list of old email addresses)
Have a look at the Merge user accounts (tool_mergeusers) plugin. If you have further requirements specific to your environment you may need to modify this plugin, or develop your own plugin to track changes using the Events API and respond to these.