I would like to report back that I was finally able to get through to a supervisor who fully understood the situation and was able to undo the changes from the previous agents. The old SIM was reactivated and the phone worked again.
She then took over in the account changes (as per the original quote) and fully honored it. In the subsequent days she followed-up and ensured the new phone was shipped and the new plan was activated, and addressed all my additional questions relating to this miscommunication between the agents.
Thank you to Annie (supervisor) and the Social team for following up and resolving all of this.