restoring to factory settings

Do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart). Run Disk Utility/First Aid. Reformat the drive using Disk Utility/Erase. Use Mac OS Extended (Journaled), then click the Option button, and select GUID. Quit Disk Utility, then re-install the OS.

Reinstall from macOS Recovery

OS X Recovery

When you reboot, use Setup Assistant to restore your data.

For changing your email address, note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

Apple ID -Two-factor authentication          

For AVG, see if anything here helps.

https://www.wikihow.com/Uninstall-AVG

https://support.avg.com/search?l=en&searchText=how+to+uninstall+AVG

https://techreviewpro.com/how-to-download-avg-removal-tool-to-remove-all-avg-products-4505/

https://search.yahoo.com/search?ei=utf-8&fr=aaplw&p=how+to+uninstall+avg+completely

You want to uninstall Malwarebytes or you have malware?

You may  have adware/malware installed. Try downloading this program which was written by Thomas R, a long time poster. The program will search for malware/adware. The program will do the work for you which makes it easy. It may be necessary to run more than one scan. After running the scans, restart the computer.

Malwarebytes Anti-Malware for Mac      10.10 and later.       

Web site for download   https://www.malwarebytes.com

Malwarebytes uninstall.   

What problem are you having with the email account? More information would help.