Cityjet has surplus capacity since cutting back at London City. Therefore in my view, as long as these routes cover their direct operating costs, it makes economic sense for Cityjet to operate them for the time being.
True - that'll be why these are starting at short notice.
I don't have the slightest doubt that an EI flight to Dublin and a KLM flight to Amsterdam would work; they definitely definitely would.
Whether or not this can work depends on marketing; but I think that if they get it right it can. However that marketing has to say "Fly from Cambridge to Amsterdam, Dublin, and a long list of places", not "Fly from Cambridge to Amsterdam and Dublin".
I think Darwin could have made their flights work if they'd let people know about them and stuck with it for a bit longer; they stopped taking bookings about the same week as they started getting ticketing arrangements in place, and disappeared about the same time as people started to get to know their name.