Easy Jet are unlikely to put additional seats in its just not suited to their business model. They are only interested in dropping in schedules that allow them to fit into their other more strategically relevant routes.
If the IOMG had thought this through they would have had a contingency plan to put into place at short notice. Its one matter losing flights from Manchester to Southampton for example but there are virtually no other alternatives in the case of the Isle of Man because of the Irish sea!
One of the easiest things right now would be to persuade Loganair to do some emergency flights they have a booking system and a spare plane sat on the tarmac at Ronaldsway ( London City is currently varied between the Saab and the ATR) surely pulling together something there would be easy. A little outside of the box thinking and there could be an immediate improvement!