Is there any way for an end user who has been assigned to an organisation to become a "super user" to allow them to access all tickets logged for their organisation
Yes. Go to the Org, edit it and look at Settings. Set them as the Primary contact. Decide what you want it to do on that same page. Update the Org.Depending on the settings that you select they will be able to see all new tickets to that Org.
Is there a way to add the Primary contact to existing tickets for an organisation?
one of the devs tells me that its all tickets of the Org.