Sadly my idea of "A-level" refers to the UK qualification, and is therefore, particularly in this day and age, far from being a gold standard.
I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight, but we're deviating massively from the point of the thread, and I really don't think it's a critical issue when considering turnbacks. In addition, I'll be honest, I'm primarily an operator rather than a boffin, so we're getting well away from my Stick Monkey comfort zone.
Bit of a cop-out I know, but will gladly hear any more techniques/advice about the flying of turnbacks, have very much enjoyed the more practical discussion in this thread.