Sorry, but the historic reason often for the use of the stand is that it is the only one that can fit a 747, nothing to do with the security. There is now facilities for three 747 stands to accomodate BA from October. You are correct that both ELAL and SAA lounges are located side by side on the pier.
Terminal 3: BA all used it for the LAGOS flight a few years ago. The rumoured reason for BA presence is its future proximimity to T5 and also its hoped for better links with AA. The staff for T3 are in the main provided by T 1. Ramp staff also go to T1 to load and unload the Finnair aircraft during the day.