Outstanding charges not visible to account holder and suspicious primary user on account

After closing an Xfinity account, I received an outstanding bill. In hopes of inspecting the billing history, I logged into the Xfinity site with my @Comcast.net username and password. Unfortunately, the billing information was not visible according to the permissions and role on the account. In hopes of correcting this issue, I contacted Xfinity customer support by phone. They informed me that my @Comcast.net username was marked as a secondary user and provided the real name and username of a primary authorized user. This primary authorized user name was not familiar to me, and I'm worried that at some point the account was comprised and a false authorized user was added. At that point, customer support routed me to billing as they were not able to mark my secondary user account as the primary, but then the billing folks just said that they couldn't help.

I'm hoping to:

• Gain access to the billing history so that I can validate the timing of the outstanding charges
• Identify when the unrecognized user was added/made primary and determine all account actions/changes made by this unrecognized user