Terminal expansion to the east
I believe you have hit the nail on the head - the intention is additional public seating (a welcome addition in many people's eyes) and the extra retail outlets shown are the inevitable result of the way smaller regional airports have to make their money these days; that and other things such a scar parking.
There have been comments on other aviation message boards about the look of the extension but so long as it's functional I suppose the outside appearance is not that much of a consideration, especially if the cost of construction is reduced. It's always a subjective opinion but I think that most of the recent additions will not win prizes in architectural contests for beauty. In many ways the airport looks like a Lego construction especially the western walkway but so long as the new structures do their job I'm not that much fussed what they look like.
easyJet Keflavik
The service began on 12 December last year with the A319 (156 seats) shown as the aircraft type in the original press releases. In the event both rotations through last winter (Thursdays and Sundays) were operated by A 320 aircraft (180 seats).
The average loads through last winter were:
December 139
January 160
February 163
March 151
From April the service continued as 2 x weekly on Thursdays and Sundays with the Sunday rotation a 319 (the Thursday remained a 320).
The average loads through this summer so far are:
April 141
May 131
June 135
July 145
All stats courtesy of the CAA
From November the service becomes 3 x weekly through the winter with a Tuesday rotation added. The easyJet booking engine suggests all flights will be on the 320.
No easyJet flights (to anywhere from BRS) are bookable beyond March at this time.
Of course, we have no idea of the most important information - the yield.