I believe that the reason Stobart was so eager to see the recommencement of scheduled services from SEN in March 2011 was in order to bolster shareholder confidence. They needed to show that the airport could 'attract' an existing operator and if that operator was willing to switch flights from an existing LON airport to SEN so much the better. Aer Arann fitted the bill ideally and Stobart used their influence as shareholders in the airline to achieve their aim.
Whether or not the presence of Aer Arann operations at SEN had much influence on the easyJet decision I wouldn't like to say; but it cannot have done the negotiations any harm.
Getting back to LTN, it's easy to say that no one took up the vacant WAT route because no airline had any spare capacity, but I doubt that is really true. It is quite possible that yields were insufficient by then to make it worthwhile for anyone else to make the effort to restart the route. There is little doubt that the route performed well until around 2010 but after that the combined effects of the overall recession and the Irish economic collapse changed the picture completely.