If it is a case where just a few drops are used essential oils generally don't need carriers which would dilute their potency but they can be mixed with each other for special effects. For general massage purposes a few drops of essential oils can be mixed with the massage oil to enhance the effects of the massage. Herbs can also be added to the massage oil but is added during the curing stage.

Cured oils have a long shelf life but I don't know exactly how long.

Ghee, which has the milk solid removed, can be used as a carrier for herbs when eaten. It is a very light oil and carries the herbs deeper into the tissues which makes them more effective.