I believe "this", considered a wrong answer, would be more appropriate than "that" in the sentence. With respect to location: "Este(s)/esta(s)/isto" is close to the person that speaks/writes (I). "Esse(s)/essa(s)/isso" is close to the person that hears/reads (you). "Aquele(s)/aquela(s)/aquilo" is distant from both the speaker and the listener (he/she). Thus, if we are eating a fruit, it would seem that the fruit is being eaten by me and you, and both of us are close together and to the fruit. In English, "this/these" is used to refer to what is close to the speaker and "that/those" to what is more distant to the speaker. In this sentence, the "nós/we" approximates the fruit to the speaker. Therefore, "this" appears more suitable than "that". In general: ESTE == THIS; AQUELE == THAT; ESSE has a nuance somewhere between THIS and THAT, and which of them to use to translate this demonstrative depends on the location of the associated noun, according to the rule above.
My 2kb - "such" in this sense seems to stand in place of "kind of" or "type of". I.e. We eat that kind of fruit. Or we eat that type of fruit. Therefore... I agree that "that" fruit should be used as a contracted form of the sentence, but that "such" (while sounding unusually cool) is fine in its place.
Yes, of course. It is like English. "We eat that fruit [during the holidays]" does not mean just one piece of that fruit. In Portugal for instance, lychee are to be found in nearly every store, but only at Christmas time. I doubt if everyone only eats just one lychee. But if they did they would probably say, "I eat [just] one of that fruit."
What it does mean, is just lychee, or just oranges, or just mangos but not all three and/or several other kinds of fruits.
However, outside of Brazil, Portuguese does use plurals more like English. So, "estas frutas" or, "Nós comemos frutas" when more than one kind of fruit is meant. Well, "fruit" is not a good example cause fruit is both a singular and a collective noun I believe in both languages, but can also be a plural noun as well.