In fact in Italian, which is close to Latin, "sentire", the root of sentient, means to feel. "Sapere", the root of sapient, means to know.

There's a higher level of intelligence required to "know", rather than to just "feel".

Is it really incorrect or does the term just happen to be a homonymous collision of more than one semantic concept?

I don't know (it's an actual question I ask myself, not some rhetoric vehicle), but the concept you declare as incorrect is so widespread it makes me wonder... (obviously, the meaning in the legal context we are talking about it's clearly on one side)

Incorrect in the specific context of a scientific or legal discussion which operates using a formalized definition. In normal conversation it is sort of nonsensical to say that the widespread usage of a word is incorrect, as correctness is defined by widespread usage.

The usage in SF stories stands out because often the writer was _intending_ for the word to be using the formalized definition used in science.

Traditionally, "sentience" includes consciousness, though. The OED defines it as "The condition or quality of being sentient, consciousness, susceptibility to sensation". It's actually science fiction that invented the concept of "sapient" in the sense you mean it -- the OED just refers to it as an old word for "wise" like Milton writing of "a sapient prince"

Well, that definition, like most dictionary definitions of complex philosophical or scientific concepts, is circular or too vague. The quality of being sentient is obviously circular, Consciousness is even more vague a concept than sentience, and Susceptibility to sensation is either circular (if we seek to define sensation narrowly) or too vague (if we define sensation broadly - after all, rocks are susceptible to sensation in some sense, as they immediately move when you kick them). This is not a critique of the OED: dictionaries are meant to explain to humans what other humans mean when they use a word, not give an exhaustive understanding of its deeper meanings and implications (encyclopedias aim for that).

When actually trying to rigorously define the word "sentience" we currently have settled on a very basic but measurable criterion: does the living thing we are discussing react to stimuli in its environment, and does it do so in a manner which suggests some kind of memory or other information processing power?

Conversely, consciousness has no similar even slightly rigorous definition, but it is still used as a "higher level" of sentience, sentience + some other stuff that bring you closer to how humans experience the world. Sapience seems to fit somewhere in between.