[Chrillepille]

I am really a novice when it comes to swimming with a monofin, so others can probably answer your questions better or comment on my answers. A lot of your questions regards speed. If you want to swim fast I think the main issue is being streamlined. Decreasing your resistance in water should have a much greater effect on speed and energy economy than the anatomy of the bones in the legs or tail. A mermaid/man will because of their outer anatomy always be a slow swimmer compared to fish and marine mammals and from an evolutionary point of view can not rely on speed for survival (escaping form predators and collecting/catching food). Their advantage should probably be intelligence and being able to construct things with their hands.

These were just my thoughts.

/Christian, who is neither a physicist nor a biologist