Learn Finnish: Jos sali on kylmä, meillä on toinen vaihtoehto. - If the hall is cold, we have another alternative.

No. Unlike some other European languages, Finnish doesn't usually invert the word order in the main clause after an "if" clause. You just write the main clause normally, which is why it is simply meillä on (we have).

Is the comma after kylmä required, or is it just a stylistic pause?

It is strictly required. In Finnish grammar, a subordinate clause (like the one starting with jos) must always be separated from the main clause by a comma.

I learned that toinen means "second". Why does it translate to "another" here?

The word toinen is used for both! It is the ordinal number for "second", but it is also the standard word for "other" or "another". You have to rely on context to tell which meaning is intended.

What are the parts of the compound word vaihtoehto?

It is made of vaihto (exchange or change) and ehto (condition). As a neat connection to this lesson, ehto is the root word for "conditional" in Finnish, which is exactly the type of sentence a jos clause creates!