"How is is even POSSIBLE that the opening balance for the reconcile isn't the closing balance of last month"
The Beginning Balance is the net of all previously Cleared entries. If any entries were changed, affecting their Cleared Status, or deleted and recreated, the Money might still be fine, but that Status is affected. Or, someone deleted what they thought was a duplicate transaction, deleting a Cleared one. If you run the Discrepancies report, this shows you Changes (it is Not an Error report).
And you an simply Ignore the beginning balance. Try reconciling to the known ending balance as usual and see if any "older" transactions show as needing to be Cleared, again. If so, checkmark them.