1. You reminded me of another nice example of wrong shortcuts. Average speed of car from city A to city B is 120km/h. Average speed back (B -> A) was 80km/h. What was the average speed of the car on the whole path (A -> B -> A)? It's not 100km/h.
Of course it is... if you're averaging over distance. There's no reason that's less valid than whichever average you have in mind.
I don't doubt you're right, but I don't see it. My calculation is as follows: suppose the distance between both cities is 120km. The trip A -> B thus takes exactly one hour. The trip B -> A takes 1.5 hours (120km / 80km/h) = 1.5h. The total trip therefore takes 2.5h, the total distance is 240km, which averages at 96km/h (240/(1 + 1.5)).
A simple (V1 + V2)/2 is surely possible, but I don't see the value of that average, I can't think of a situation where it would be useful.