Le montagne russe sono l'attrazione più veloce.

Because montagne russe is a feminine plural expression, so it takes the feminine plural definite article le.

• le = the for feminine plural nouns
• montagne is the plural of montagna
• So the article has to match: le montagne russe

In Italian, le montagne russe is a fixed expression. Literally, it means Russian mountains, and it is normally treated as a plural noun in grammar.

So even when English might say the roller coaster, Italian often says le montagne russe.

That means you usually get plural agreement:

• le montagne russe sono...
• vado sulle montagne russe

If you want a more clearly singular countable noun, Italian also has l'ottovolante.

Because the subject, Le montagne russe, is grammatically plural.

In Italian, the verb must agree with the grammatical form of the subject:

• la casa è
• le case sono
• le montagne russe sono

Even if the expression refers to one attraction in meaning, it is still usually treated as plural in grammar, so sono is the expected form.

Because l'attrazione più veloce means the fastest attraction, and here Italian is identifying le montagne russe as one attraction.

So the sentence is basically saying:

The roller coaster is the fastest attraction.

Even though le montagne russe looks plural grammatically, it can refer to a single ride/attraction as a fixed expression. That is why the complement can be singular:

• sono l'attrazione più veloce

This may feel odd to an English speaker at first, but it is normal once you remember that montagne russe is a special expression.

Because veloce is describing attrazione, not montagne russe.

The phrase is:

• l'attrazione più veloce = the fastest attraction

Since attrazione is singular feminine, the adjective must agree with that noun.
For veloce, the singular form is the same for masculine and feminine:

• masculine singular: veloce
• feminine singular: veloce
• masculine plural: veloci
• feminine plural: veloci

So più veloce is correct because attrazione is singular.

Because many Italian adjectives have the same form in the singular for both masculine and feminine.

Veloce is one of those adjectives:

• ragazzo veloce
• ragazza veloce
• ragazzi veloci
• ragazze veloci

So in l'attrazione più veloce, the adjective really is agreeing with a feminine noun; it just happens that the feminine singular form is also veloce.

It is just la shortened before a vowel.

• la attrazione becomes l'attrazione

This shortening is called elision and is very common in Italian.

Other examples:

• l'amica = la amica
• l'isola = la isola

Yes, più literally means more, but with a definite article and a noun phrase it often creates the relative superlative:

• più veloce = faster / more fast
• l'attrazione più veloce = the fastest attraction

So in this sentence, più veloce is part of a superlative structure:

• the + noun + most + adjective
• l' + noun + più + adjective

That is why the whole phrase means the fastest attraction, not just a faster attraction.

Not if you want the meaning the fastest.

• più veloce = faster / fastest depending on context
• velocissima = very fast or extremely fast

So:

• l'attrazione più veloce = the fastest attraction
• un'attrazione velocissima = a very fast attraction

These are not the same.

Usually, le montagne russe is the normal everyday expression.

A learner may expect a singular form like la montagna russa, but that is not the most common standard way to refer to the ride. Italian normally prefers the fixed plural expression le montagne russe.

If you want a clearly singular countable noun, l'ottovolante is often a better choice.