It depends on the team name. Some are singular, some are plural. 'The Vancouver Canucks' is plural so you would say that they are a team from Canada. The Miami Heat is singular so it is a team from Florida.

The way you would treat it is the convention in British English. The Beatles were one of the biggest bands ever... Manchester United are one of the most famous football teams in the world...The Miami Heat play on Thursday against... Treating something like a sports team as a singular entity is something that is a feature of American English.

That's tricky. The same thing can occur when discussing bands/music groups. Offhand, I would say use the singular form for the sake of subject-verb agreement, but both are possible, and as you have seen on wikipedia, both are in common use in standard English. It is important to note here, and with many other tricky examples like this in learning any foreign language, that it is less important to be 100% "correct" in the traditional sense, and more important to match how native speakers commonly use their own language. After all, how could a whole community of speakers all doing the same thing ever be "incorrect" about their own language? Edit: on second thought, I am more likely to say "The Miami Heat compete..." than "competes". However, I would say, "The Miami Heat are a basketball team" and not "The Miami heat is a basketball team."

There tends to be a difference in American and British usage in this regard, so AmE tends (it's not absolute, as you noticed) to use singular verb forms where BrE favors plural. But I also suspect that in the context of basketball teams (or Am football, or baseball), this tendency is mitigated by the predominance of plural names ("Warriors", "Clippers", etc.). I think you might notice this effect more for country team names, thus "USA is", but "England are".

This is very tricky in English, as it is based on context. If the individuals that make up the group are acting in unison or as a group, then you use the singular form of the verb and pronouns should be singular. If the members of the group are acting as individuals, then you use plural. For example, using the collective noun "jury": When the judge enters the court room, the jury stands to show its respect. (The group acts as one = singular verb and pronoun) The jury disagree about verdict and tell the judge that they are deadlocked. (The members of the group are not acting as one = plural verb and pronoun) In reality, especially spoken English, people use whatever sounds best.