The main limiting factor in the useful lifetime of the ink is chemical reactions between the components. Evaporation of solvents is less of a concern if the ink is stored properly (capped and upright at 4-10C), but over long periods can cause the ink at the tip of the nozzle to dry out - to help prevent this from happening, I generally recommend purging a small amount of ink every few weeks to maintain fresh ink in the tip.
You are right that there are some approaches which can slow down the progress of chemical reactions in the ink. However, one of the realities of designing functional materials is that the quantity and properties of every component will have an impact on the final properties of the ink. The formulation we currently use is a careful balancing act of components used to achieve excellent solderability, conductivity, and mechanical properties of the cured ink, and the proper rheology so that ink can not only be dispensed easily, but maintain its shape once dispensed.
The procedure that we currently recommend (store upright and capped, refrigerated at 4-10C) is the best way to ensure that the shelf life of the ink will be maximized. Almost all materials (even solder pastes) have an expiry date, and good storage practices are the best way to maintain the maximum shelf life.
Expiry of the conductive ink is a complex process, and it is difficult to predict exactly when a cartridge of conductive ink will stop performing suitably. The expiry date we list on the cartridges is based on our experience and testing thus far, but how long the ink will last after the expiry date depends on the storage conditions. For example, check out my post on this thread - the ink I tested was stored in a sealed cartridge at recommended conditions, and it both printed and soldered well past the expiry date.