Using a query and update in a Page Reference

Something like this might be helpful. Have the PageReference method call the List<CnP_PaaS_EVT__Event_attendee_session__c> method... This way the list method can still be called from other places in the code.

public PageReference saveAttendee(){
        CnP_PaaS_EVT__Event_attendee_session__c attendee;
        // use for loop to get the first result, then break out of the loop.
		for (CnP_PaaS_EVT__Event_attendee_session__c firstResult : attendeeListFor(registrantId, ticketLevel) {
			attendee = firstResult;
			break;
		}
        // unable to proceed if attendee is not found...
		if (attendee == null)
			return null;
        attendeeId = attendee.Id;                                                              
        attendeeFirstName = attendee.CnP_PaaS_EVT__First_name__c;
        attendeeLastName = attendee.CnP_PaaS_EVT__Last_name__c;
        mailingCity = attendee.DTCI_Mailing_City__c;
        mailingCountry = attendee.DTCI_Mailing_Country__c;
        mailingCounty = attendee.DTCI_Mailing_County__c;
        mailingState = attendee.DTCI_Mailing_State__c;
        mailingStreet = attendee.DTCI_Mailing_Street__c;
        mailingZip = attendee.DTCI_Mailing_ZIP_Postal_Code__c;
        update attendee;                                        
		PageReference sendToReleaseForm = new PageReference('/apex/DTCI_Release_Form_Page_AB');
		sendToReleaseForm.getParameters().put('attendeePageId', attendee.id);
		sendToReleaseForm.getParameters().put('registrantPageId', registrantId );
		sendToReleaseForm.getParameters().put('eventPageId', eventId );
		sendToReleaseForm.getParameters().put('releasePageType', attendee.DTCI_Release_Type__c);
sendToReleaseForm.setRedirect(true);                        
        return sendToReleaseForm;     
    }
    // this method still returns a list, but your PageReference method can call it...
	public List<CnP_PaaS_EVT__Event_attendee_session__c> attendeeListFor(String registrantId, String ticketLevel) {
		return [SELECT Id, Name, DTCI_Agreement_Active__c, CnP_PaaS_EVT__First_name__c, CnP_PaaS_EVT__Last_name__c, DTCI_Ticket_Level__c, CnP_PaaS_EVT__EventId__r.CnP_PaaS_EVT__category__r.name
				   FROM CnP_PaaS_EVT__Event_attendee_session__c                                                                            
				   WHERE CnP_PaaS_EVT__Registrant_session_Id__c =:registrantId 
						AND DTCI_Ticket_Level__c =: ticketLevel           
						AND DTCI_Attendee_Info_Assigned__c = FALSE
				   ORDER BY DTCI_Ticket_Order__c
		  ];
	}