Hi, I am heading to Disneyland Paris in June and have been looking at transport options. Can the 10 journey RER tickets (carnet) be used in zone 5, to get me from Torcy to Disneyland? Thank you

You can purchase a booklet of ten RER tickets specifically from Torcy station to Marne La Vallee / Disneyland if that's your only destination.  RER ticket prices & validity are not based on zones per se, but from station to station (although RER tickets are valid for travel to any station that is closer than the destination listed on the ticket).
For example a ticket from Torcy to Marne la Vallee is 3.55€ while Val d'Europe to Marne la Vallee is only 1.95€, all stations are within zone 5.