I will try to help. Chaff Delta is fired for distraction. Making a nice big false target, but it not effective at high speed as you want to stay in the pattern. THe scale of your map is a bit misleading Exeter was a about 25 miles south of the main task group and the T21 a further 10 south of her, so well within Sea Dart's envelope. EMCON state would depend on the perceived threat the time. The Etendards and the Exocet would have been detected by both ships passive systems which would give an approximate bearing of the threat. No point in staying EMCON silent once you been detected and fired at. Exeter behaved exactly as she should picking up the raid and taking out two.
I might be hepful to watch this documentary at 18:00 minutes in.
Shame you are so far away. My collection of book on the conflict are now available. Probably the finest collection anyone could want.. Im moving kindle which is easier on aging eyes.