The name of the third planet from the Sun is "Earth". In particular, its name is not "Terra".
Don't take my word for it, of course. Just look it up in the dictionary.
That's fiction. If Earth is called "Terra" in a work of fiction, that just means that's its name within the setting of the story. In the novel A Princess of Mars, Mars is called "Barsoom", but this in no way means that this is really Mars's name.
Fiction authors are generally well aware of this distinction. Earth is called "Terra" in Arthur C. Clarke's novel Imperial Earth, but when not writing fiction, Clarke always referred to it as "Earth". See, for instance, his textbook Interplanetary Flight.
No, that doesn't apply to proper names. Specific people, places, and things do have official names, which can't be changed by common consensus. If we all started calling you "Spot", would that mean that your name is "Spot"? Of course not.
Also, even if that were the case, nobody actually calls it "Terra" outside of fiction, except science fiction fans in informal settings. In formal nonfiction publications that have undergone editorial review, it's called "Earth". Some examples:
No they do not: example 1, example 2, example 3.
The International Astronomical Union, an internationally recognized group of professional astronomers. And they say:
The eight planets [in the Solar System] are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
True but that doesn't matter, since I'm talking about the English name for Earth. Other languages have other names. The Italian word for Japan is "Giappone", but that doesn't mean the actual name of Japan is "Giappone" when you're speaking English.
True but that doesn't matter. "Cardiac" means related to the heart, but the word is still "heart", not "cardia".
Not in the real world, no.
Fine. I don't care. Just don't tell anyone else that its name isn't "Earth".