Bagso
You raise a lot of fair points and I certainly agree that MAG faces a lot of challenges in earning an attractive return on its investment.
But MAG's strategy is pretty clear. They want to kick start growth by doing deals with the incumbent carriers. Even though this involves discounting airport charges MAG will have calculated that there will be an overall increase in revenues due to higher volumes and also to commercial revenues from these extra passengers. Because STN has oodles of spare capacity MAG doesn't need to invest in new facilities to handle this growth in traffic, for the medium term at least. Secondly they want to boost commercial revenues, for example by the works to expand the departure lounge to create more retail facilities, at the expense of the underused check-in area. Thirdly they want to reduce operating costs, by a combination of running the airport more efficiently than BAA did, and sharing some support functions with MAG's other airports.
The theory is that all of this will lead to a rapid increase in profitability. Time will tell. I don't have inside info so I can't answer your question as to how STN's profits at 20m pax will compare with MAN.
On some of your other points.
Yes, LCC pax often spend less (on average) than other pax. But this is not always the case - some pax respond to having got a great deal on the air fare by splurging on things like lounge access etc. And airports can tailor their offer to LCC pax, for example by better 'grab and go' catering.
On landing fees, STN is now technically able to charge what it likes, subject to general Competition law. But the airport has only been removed from regulation because it is thought that the airlines now have enough power to stop the airport from putting up charges, for example by threatening to reduce traffic.
On synergies, I agree that these are limited, at least in relation to airport operational activities. But I'm sure there are a range of support activities (Board, marketing, procurement, insurance, planning, engineering support etc etc) that can be shared across MAG airports, reducing overall costs.
As I said earlier, time will tell as to whether MAG's £1.5bn gamble has paid off. But one thing is certain - by the time we know whether it's a good or bad deal all those responsible will have long gone, spending their bonuses etc!