We already know from TOS that Mutlitronic computers are able to develop sapience, with the M-5 computer being specifically designed to “think and reason” like a person, and built around Dr Daystrom’s neural engrams.
However, we also know from Voyager that the holomatrix of their Mk 1 EMH also incorporates Multitronic technology, and from DS9 that it’s also used in mind-reading devices.
Assuming that the EMH is designed to more or less be a standard hologram with some medical knowledge added in, it shouldn’t have come as a surprise that holograms were either sapient themselves, or were capable of developing sapience. It would only be a logical possibility if technology that allowed human-like thought and reasoning into a hologram.
If anything, it is more of a surprise that sapient holograms like the Doctor or Moriarty hadn’t happened earlier.
In this case, ‘sapience’ is correct. However, I don’t recall them ever making a distinction, in-universe, between the two.
The main difference between sentience and sapience is self-awareness. A sentient being has consciousness, the capacity for sensation, and a subjective experience. Many animals can be described as sentient, although it’s hard to know for sure what’s going on inside a fish’s head. Sapience, on the other hand, is marked by a higher level of cognition and intelligence. Human beings are sapient creatures.
Not as far as I’m aware. TOS did seem to make more of a distinction between the two, but TNG treated them as one and the same, with most other series following suit.