Unser Gehöft ist abgebrannt.
Ist abgebrannt can be understood in two different ways that both give the same meaning:
The stative passive is used for stating the state something is in now — a state whose creation the subject endured passively at some point in the past. Since the farm very likely didn’t burn itself down, it has endured a past passive action which is implied by the stative passive.
If a combination of adjective and copula is assumed, one can say even less except that the farm is now in a burnt down state.
Neither of these two construction state anything about the cause of the burning; whether it was arson or accident or God’s revenge. And I would actually allow the following two translations, the former of which placing more emphasis on the burning down having happened in the past, the latter of which placing more emphasis on its current state of being burnt down:
Our farm burnt down.
Our farm has burnt down.
To imply arson, as the following English sentence does, one would need to use a dynamic passive.
Our farm was burnt down.
The dynamic passive is the standard English passive form, and in German it is rendered with the modal verb werden. To shift tenses, a corresponding form of werden needs to be retained in the sentence. Thus, the following translations are correct:
Our farm is being burnt down. — Unser Gehöft wird abgebrannt. (or: niedergebrannt)
Our farm will be burnt down. — Unser Gehöft wird abgebrannt werden. (or: niedergebrannt)
Our farm was burnt down. — Unser Gehöft wurde abgerannt. Or: Unser Gehöft ist abgebrannt worden.
Our farm had been burnt down. — Unser Gehöft war abgebrannt worden.
: There is no difference in meaning between preterite and perfect forms in German. The further North you go, the more likely you are to encounter preterite, which is also often the preferred form in written German.
You can see that all your assumptions are correct.