An excellent post Sotonsean, just to add to the extensive info you have already posted, pretty much every US carrier that flew schedules to the UK has flown to Gatwick at some point - the one exception being United who only ever had a charter series to Gatwick prior to starting Heathrow in April 1991.
In addition to your list
American Airlines took over Braniff's Dallas/Ft Worth in May 1982 following their collapse, in addition they bought Continentals Miami route in 1989/90, which was previously Easterns and just prior to starting Heathrow in July 1991 had started flights to Chicago, the route authority purchased from TWA but having to start ops at Gatwick until they got Heathrow authority. Over the years in addition they have also flown Boston, New York JFK, Nashville, Raleigh/Durham and St Louis from Gatwick, the latter after the TWA takeover.
Delta flew to Boston and took over Pan Ams Miami route in 1991, I don't think Continental ever flew Boston Gatwick.
TWA also flew to Pittsburgh in the 1980's, it also moved Philadelphia from Heathrow in July 1991 as this was the only Heathrow route not sold to American as its sale was blocked by the US authorities, this was flown from Gatwick until sold to US Air along with Baltimore.