I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight
Erm, absolutely no! Speed with respect to the airmass inertial frame of reference is
entirely what it's about.
I really don't think it's a critical issue when considering turnbacks.
Erm, absolutely no again! Significant loss of (positive) airspeed in the turn, 'cos of induced drag for relatively slower-rate turns by conventional fixed wing aeroplanes or inertial effects for rapidly-turned Harriers/helicopters moving slowly with respect to the ground, is one of
the major factors going against the attempted turn-back and which, for conventional fixed wings, can result in the stall/spin fatality. In response to earlier posts, however, this is not a 'turning downwind' phenomenon. Exactly the same occurs turning upwind.