Your first pic showing Tarzan's left leg's transition from end of kick upbeat to start of kick downbeat, exemplified what I sometimes prattle on about - the lower leg bent back, possibly as much as 30 degrees, the sole of the foot horizontal -
and because of the knee bend, some of the shin area being used for a good (if momentary) push back - aided of course by flexible feet. Thorpe does the same - in fact is this now a common feature of sprint swimmers? (This is like the pics of Morozov's leg action in Sheila Taormina's "Swim Speed Strokes".) The other point of interest is the fact that the thighs barely come in front of the torso line (This seems to apply to most speed swimmers now - all the leg action being above the body line, re your comment about minimizing drag in the water flow passing under the body!). This is what I saw on my GoSwim Lezak DVD I have - he also swims with his torso tilted up very slightly - hydroplaning? - and you get the impression
he tries to keep any disturbance of water above the body line - with a smoother flow beneath it! Surprised by "Tarzans" fairly late high elbow start of pull in that first pic, but I think, from memory (without looking things up) Sheila has pics in her "Swim Speed Secrets", showing he initially uses his curled over fingers to "grab the water" as an arm enters, and
then straightens the hand as it goes down to a proper catch. I think Sheila has great estimation for his catch technique since she has several pics of his (seemingly better) arm action in her other book. IMO Tarzan's more continuous arm action was absolutely essential for his choice of keeping the head high - a bit like polo swimmers - but it certainly keeps
both legs and arms working together in an optimal fashion (without any drop-off in propulsion from non-propulsive arms while only the legs are acting) - however, rather tiring for a distance swim!

Does this all mean we ought to strive for a slight upward curl to the body when trying to go a bit faster (without allowing the belly to sag - that would cause extra drag for some of us!

)???