Među našim prijateljima nitko ne voli dosadne filmove.

Because među can go with different cases depending on the meaning:

• instrumental for a static situation: među prijateljima = among friends
• accusative for movement into a group: među prijatelje = among/to the friends

In this sentence, we are talking about people within a group, not moving into it, so Croatian uses the instrumental: među našim prijateljima.

Because the noun has to match the case required by među in this sentence.

The noun prijatelj changes form by case:

• nominative plural: prijatelji
• instrumental plural: prijateljima

So našim prijateljima is the correct form for among our friends.

In Croatian, this is normal and required. Croatian uses negative concord, which means that if you use a negative word like nitko (no one / nobody), the verb is also negative:

• Nitko ne voli...

This is standard Croatian grammar. English usually says Nobody likes... with only one negative element, but Croatian normally uses both.

Because nitko is grammatically singular.

Even though it refers to zero people, Croatian treats nitko like a singular subject, so the verb is in the 3rd person singular:

• nitko ne voli = correct

So voli is singular because it agrees with nitko.

Because this is the direct object of the verb voljeti (to like / to love), so it goes in the accusative plural.

Compare:

• nominative plural: dosadni filmovi = boring films
• accusative plural: dosadne filmove = boring films, as the object

The adjective dosadne agrees with filmove in case, number, and gender.

Yes. Croatian word order is fairly flexible.

This version begins with Među našim prijateljima to set the frame first: as for our friends / within our group of friends.

You could also say:

• Nitko među našim prijateljima ne voli dosadne filmove.

That is also natural. The meaning stays basically the same, but the emphasis changes a little.

They are related, but not the same.

• među = among, in the middle of a group
• između = between, in the space separating things or positions

So in this sentence, među našim prijateljima means among our friends, which fits the idea of looking at a group. Između naših prijatelja would not sound as natural here.

Yes, absolutely. That is a very natural sentence too.

The difference is mostly one of focus:

• Nitko od naših prijatelja ne voli dosadne filmove. = None of our friends likes boring films.
• Među našim prijateljima nitko ne voli dosadne filmove. = Among our friends, no one likes boring films.

The version with od naših prijatelja feels like selecting from a set. The version with među našim prijateljima emphasizes the group as a whole.

Because Croatian has no articles.

There is no separate word that directly corresponds to English the or a/an. Croatian usually leaves that information to context.

So prijateljima can mean friends or the friends, depending on context, and dosadne filmove can mean boring films or, if the situation makes it clear, the boring films.