5) You must also share the agenda (all permissions) with the service account user/email (created at step 2): For this, go into Google calendar, choose "Settings and Sharing" from the combo list on the right of your Agenda on the area "My Calendars" (It must be same Agenda than the one chosen on parameter "ID Agenda" at step 1). Click on link "Share with Specific people". Then add the service account user into the email field, choose permission "Make changes to events" and validate.
With Google Apps for work (not required for standard gmail account), permissions may be forced to "Ready only" instead of "Edit". To solve this, go onto