Have there been any successful lawsuits (plaintiff won monetary
damages)in which a former employee sued his ex-employer for
unauthorized opening of employee's personal US mail addressed
personally to the employee at the employer's business address?  Does
the US Postal code provide any
specific penalties (fines, jail?) against an employer for engaging in
the unauthorized opening of an employee's mail?

i don't know, but i seriously doubt it.  all of the companies i have
ever worked for will respect a letter if it is clearly marked
"personal", but my understanding from talking with friends is that it
is not uncommon in some companies for the mailroom to open ALL mail no
matter how it is marked.  i doubt the practice would be that common if
it was illegal.

even if it was, i doubt you would get any money out of it.

-cab

What about a circumstance in which an employee was fired, subsequently
rather than forwarding that mail, the company opened the mail which
was marked to the addressee (former employer) and then used
information in that correspondence to file a lawsuit against that
former employee, based upon misinterpreting information in the
correspondence which could only have been known by opening said mail?

i just don't know enough to even guess.  since your question is about
to expire with no answer, i'd suggest either bumping the price and see
if you get a nibble, or - and this would be free! - why not call the
post office and ask?  i bet if you go to usps.com, there's a "contact
us" or similar link that will eventually put you in touch with someone
who knows.

good luck.

-cab

Just to let you know that someone else agrees with Barneca.  Ask the
Post Office.  If a law suit has been started on the base of the
misinterpreted contents of the letter, you will need to see a lawyer 
- sooner than later.

Anything posted here is not a substitute for legal advice, as you can
read in the discalimer below.