Your barcode is used to find your MacID, not to change it
1., 2. You aren't changing you Mac ID, you're just changing the password associated to it. Your barcode ID will only tell you what your Mac ID is.
3. Once you change your Mac ID password, your email password will automatically change by itself since it is linked to your Mac ID.
4. You do not need to change your barcode ID. You barcode ID is used solely to find your MacID in the event that you do not know it. Even if the hacker has your barcode ID, they won't be able to retrieve the password solely with the barcode ID
How did you even get hacked before school started? Also have you set recovery questions yet? Because if you have and you remember the answers you can reset your password yourself
here. Would highly recommend you contact McMaster's Technology Services Department to sort this out or if you are unsure about anything.