Given the differences between Etruscan civilisation and other Italian peoples of the same period, is the conclusion that Etruscan civilisation was autochthonous sensible?
It depends entirely on your definition. Before the Etruscans there probably lived some other group of people in Tuscany, and before them another.
If you consider where Etruscan culture developed as we know it, Wikipedia has a fairly good answer:
Culture that is identifiably Etruscan developed in Italy after about 800 BC approximately [in the area] of the preceding Iron Age Villanovan culture. The latter gave way in the 7th century to [an Etruscan] culture that was influenced by Greek traders and Greek neighbours in Magna Graecia, the Hellenic civilization of southern Italy. — Wikipedia on the Etruscan Civilization.
However, it is unknown where the Etruscan people came from genetically, or at least not their typically Etruscan genetic origin (they no doubt mixed blood with various other groups in the area). There are two hypotheses (from Wikipedia on Etruscan Origins):
They developed out of the Villanovan culture that was in that region before them. Their ancestors were prehistoric people that had been in Europe for a long time, though their precise movements are unknown.
They migrated to Italy from Anatolia.
Recent DNA and mitochondrian-DNA research provide some tentative evidence for the migration hypothesis: various genetic similarities have been found between Etruscan bones, modern Tuscans, and modern Tuscan cattle on the one hand, and people and cattle in Turkey and the Caucasus on the other. — Wikipedia.
The term "autochthonous" basically means the same as indigenous. Therefore, I interpret your question as asking if it is valid to claim that the Etruscan Civilization could be considered to be indigenous to the portion of Italy where they resided. The answer to that would be "yes".
The Estruscans were a unique civilation that resided in the northern part of Italy. they had their own language, their own forms of government, and their own distinct culture. According to wikipedia, they had existed for about 600 years before they became a part of the Roman Republic.
Roman mythology tells the story of Romulus and Remus and how they founded Rome. The manner in which they selected the location appeared to follow Etruscan tradition, and the name Rome is believed by some to be Etruscan.
It is my understanding that genetic research now tentatively supports Herodotus's claim (previously considered just another of his wild concoctions) that the Etruscans had originated in Asia Minor. Moreover, the same is true of Tuscan cattle as opposed to cattle in other parts of Italy.
The Lemnos stele inscribed in a language very similar to Etruscan does not contradict this theory, either, given the position of the island of Lemnos.
The origin of Etruscan civilization is one of the more mysterious historical enigmas that is with us to the present-day.
It is very likely that many of the present-day peoples of the Tuscan region in Central Italy, are of distant Etruscan ethnic descent. Although the Romans conquered the Tuscan region 2300 plus years ago, there is no historical evidence which suggests or explicitly states that the Etruscans entirely disappeared as a people. That is to say, the Etruscan language and culture disappeared, due to, (in all likelihood), forced assimilation/(Romanization). However, the Etruscans, as an actual people, most likely survived the Roman colonial onslaught, though have been fully Romanized/Italianized since the 300's BC/BCE.
There are some plausible theories which suggest that the Etruscans may have distant Greco-Anatolian ethnic origins and that Ancient Tuscany, may have been distant member of the Magna Graecia cities and regions. Etruscan Art, does have some quasi-Hellenic influences and even aspects of the (now defunct) Etruscan language, may have distant Hellenic roots.
The Etruscans may have been a Latin tribe distantly related to the Romans, though there appears to be little historical and linguistic evidence that is available for confirmation.
Overall, the ethnic origins of the Etruscans continues to remain a mystery to historians, linguists and archaeologists. Perhaps with advancements in archaeological technology and the study of Indo-European languages, more information regarding Etruscan ethnic genealogy will be available at a future date.