Re: who were creating/commas
None of the sentences is very natural. Try to keep the relative pronoun as close to its antecedent as possible:
Last week, three men who were creating a drunken disturbance were arrested in our small town.
It's up to the speaker/writer to decide whether a restrictive or non-restrictive clause is appropriate.
In your other example, only a non-restrictive clause is appropriate:
Last week, they arrested Jack Smith, who is the mayor's son.
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