HAC - your sums seem fine to me and it looks like BE will have a very busy, and presumably profitable, few months on LGW. Even after October, when EZY seats are on sale, BE's entry level fares have moved up significantly from last winter and the very cheapest fares are no longer available on some flights, on some days.
What to do about overall capacity in the summer months is tricky though.
There are a handful of extra flights in the system for TT period. As IOMspotter has said Luton has been reduced from 2011, so if they squeeze the same number of PAX into fewer flights, that route should do well too, with few empty seats to play with.
They could put a 175 or 195 into IOM, but that would presumably come with some start up and training costs, plus a loss of operational flexibility, with a mixed fleet, and a different ball game next winter anyway, once EZY start flying.
My guess is that BE will just fill their boots with increased loads, and significantly enhanced yield, from all those ex-LCY pax, shed price sensitive leisure pax, who will go to EZY eventually. They'll then see how things play out over the next 12 months or so, before making any material changes to the way they run the route.