The name Etruria was likely chosen by Napoleon due to his well-documented fascination with the classical world, particularly Roman civilisation, and his desire to emulate the Romans in his policy. Reading the classics while at school in Brienne, first proclaiming himself Consul then Emperor, invading Egypt in part to emulate Alexander the Great, and the famous paintings "Napoleon Crossing the Alps" which were produced from 1801-5, with references on stones in the corner comparing Napoleon to Charlemagne and Hannibal. The man was obsessed with all things classical, introducing golden eagle standards for his military corps and giving various Latin names to his newly-created client states: the Cispadane, Transpadane and Cisalpine republics in northern Italy, and the Batavian republic in the Low Countries for instance. To him, Etruria was likely the logical choice in recreating Europe in line with a golden, bygone age.

In reference to the second part of your question, the locals' assent wasn't required - the French army was vast, drawing on almost all of Europe at its height for its manpower; they were more than capable of crushing any Italian resistance. The locals' needs were almost a non-issue when compared to Napoleon's dream of a Romanised Europe.

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Andrew Roberts, "Napoleon the Great" (UK) or "Napoleon: A Life" (US) is an excellent biography which covers in detail his obsession with the classical. The other modern front-runner is Adam Zamoyski, "Napoleon: The Man Behind the Myth" (UK) or "Napoleon: A Life" (US). They don't make their titles easy to navigate unfortunately!