"Unprovisioning (your web application name) already exists under the parent Microsoft.SharePoint.Administration.SPWebService named "". Rename your object or delete the existing object."
Fix: You need to delete the Timer Job Definition called "Unprovisioning Web Application (your web application name)". This is because SharePoint is still trying to delete the Web Application from your unsuccessful steps before.
To delete this Timer Job Definition, go to Central Admin -> Monitoring -> Review Job Definitions -> Paginate to the Job Definition called "Unprovisioning Web Application (your web application name)". Click on it and select Delete.
Now you can delete your Web Application successfully.