My basic grasp of mathematics tells me that per sector there will be an additional 5 seats.
My basic grasp of reality tells me that it doesn't matter how many seats there are on a specific aircraft, that aircraft can't be in two places at the same time. How are the Dorniers going to service the sectors mentioned? I'm not aware of this being explained in any detail in the public domain. Perhaps you can provide some details?
I remember Air Sarnia, and am aware of some of the problems, trying to run a service with one aging Islander was a little ambitious.
As for the marina plans, the current ones are a bit pie in the sky as they rely on new houses being built on the green site to generate the finance to build the marina, after reading something in the local paper it is alleged that the company proposing this development have something like £15k in the bank. Their status as preferred contractor runs out at the end of this month. I agree with you in regard to viable travel links but it has become a bit of a 'chicken and egg' situation. I disagree with you in regards to demand, I've over thirty years professional experience in the maritime industry within the Bailiwick of Guernsey and have also sailed/raced fairly extensively, from experience of the new French marinas that have been built over the last couple of decades along the Brittany coast they are all bustling places which have generated local business in terms of chandleries, restaurants and the like. As an example in Normandy, look at the amount of property built in Carteret during the last decade or so and the prices being charged for them. Geographically, Alderney is ideally placed at the crossroads for vessels cruising into the Bay of St.Malo and a protected marina would be well utilised.