Are there mistakes in those Spanish sentences?

Are there mistakes in those Spanish sentences?
 

"No vivo en una ciudad, vivo en un pueblo. Mi pueblo es muy diferente de Buenos Aires. Es en el interior del país y no tiene puerto como Buenos Aires. La mayoría de los habitantes de Buenos Aires son de ascendencia europea, pero la mayoría de los habitantes de mi ciudad son descendientes de la India."

"Al norte, el Brasil y la selva amazónica sepabaran la Argentina de los países vecinos. Al oeste, las montañas de los Andes separaban la Argentina del Chile."

"La mayoría de los inmigrantes a la Argentina son de Italia y de España. Unos son de Polonia, de Rusia, y de Alemania."


The words in red are those that I am not sure I need to include.

Let's see what I can find:

I've marked some possible alternatives (but what you wrote is fine), what is wrong and why. I also won't be surprised if I missed something.

Yes, you need to include them. The general rule in Spanish is that one repeats a preposition with each individual item in a list. Omitting the preposition strongly suggests that the speaker considers that the item and the preceding item are one thing rather than individual things. So, saying "son de Polonia, Rusia y Alemania" refers to one country whose name consists of the several words, "Polonia, Rusia y Alemania".

For example, if someone is from the Carribean country St. Kitts and Nevis (San Cristóbal y Nieves), one says "Es de San Cristóbal y Nieves", even though St. Kitts is one section of the country and Nevis is the other section of the country. One would say "de San Cristóbal y de Nieves" only when one is considering each region individually rather than the country as a whole.

"La mayoría de los inmigrantes a la Argentina son de Italia y de España. Unos son de Polonia, de Rusia, y de Alemania."

That "unos" doesn't sound good.
Trying to find out an explanation, I think we use "unos" as a pronoun when we are going to say "otros". For example:

Unos son de Italia y otros de España.

As you have already talked about some migrants in the first sentence, I would write "Otros son de Polonia, ..."