I can't speak for the Italian - I hope someone will.
In English, though "furniture" and "furnishings" are not exactly synonymous. "Furniture" involves chairs, tables, sofas, stools, etc. "Furnishings" can include furniture, but may also refer to curtains, carpets, etc. (See http://www.oxforddictionaries.com/us/definition/american_english/furnishing )
Yes, in Italian more than one piece of furniture is plural so you need a plural verb. In English, the concept is different--100 pieces of furniture is still one group of furniture, so the verb is singular. It is what I love/hate about learning another language--it is interesting to think differently, but it is also challenging.
because furniture is not countable. you can't say one furniture, two furnitures. if you want to count furniture you can use one piece of furniture, two pieces of furniture i think the italian word la mobilia is uncountable too. mobilia is maybe a better translation for furniture
Do you think all translation is word for word? If you come to my town and ask where is the furnitures, we will ask you where are you from, because it is clear English is not your first language. Italian and English speakers think of these concepts differently, so that has to be taken into consideration when making a translation.
Sono is used because the Italian translation of "the furniture" is plural ("i mobili") unlike in English. Furniture is a mass noun in English and is treated as singular from a grammar perspective (ie. We say the furniture is here, not the furniture are here). But because Italian thinks of furniture as plural you must use the plural form "sono" instead of the singular "è"