Is this accurate? It seems a bit clunky:
"I was referred to this particular job opening by my supervisor John Smith, the Human Resources representative for XYZ Law Firm."
You need a comma after "supervisor". Otherwise, it's a long sentence, but correct.
Yes, that seems right to me. "I was referred... job," is a simple subject-verb-noun construction, "by... supervisor" is an adjective prep phrase describing "job opening," "John Smith" is the essential appositive, and the rest is nonessential appositive (note that it's separated by a comma).
Reed is correct. Add a comma after 'supervisor.'
Yes, the sentence you provided is accurate grammatically, but it can be simplified for better clarity. Here's a revised version:
"My supervisor John Smith, who is the Human Resources representative for XYZ Law Firm, referred me to this job opening."
This revision maintains the same information but presents it in a clearer and more concise manner.