As far as the viability of any particular route is concerned, my experience is that it is almost impossible to accurately predict the success or otherwise of any but the most obvious candidates, unless there is recent historic evidence to go by. Sometimes an unlikely link between two city pairs can achieve commercially acceptable returns while another may look more promising, yet fails to perform. There are so many factors involved that sometimes you just have to try it and see.
In the case of CAX-SEN at least the costs of establishing the route can be minimised due to the dual ownership of the airports by Stobart and its shareholding in Aer Arann.