The Gatwick type scenario is often refered to as a "Piggyback missed approach". In theory in the UK the arrival should not cross the threshold until the previous departure is at least 2000m down the runway (if airborne, 2500m if not) but when under pressure to move the traffic errors do occur. The worst I ever saw invloved a B747 rolling and a DC10 going around .5nm behind it; it went straight down Crawley High Street and the phone lit up.
The missed approach is more likely to outperform the departure and so that is normally the one you turn; 90 degrees is ideal, 45 is probably the minimum.
Simultaneous Parallel departures are not permitted under ICAO unless the initial departure tracks diverge by 15 degrees immediately after take off.
Best of luck.
Point 4