1)What protection does a 12 month preliminary protection for a utility
patent really provide - when someone is copying my invention? What
possibilities would someone have to work around (f.e file an
application
for themselves?).

2) After having filed for preliminary protection for a utility patent,
until what time do I have to file the application for my utility
patent (is the last day at the end of the 12 months still valid?)

3) I suppose the preliminary protection does not require me to file. I
suppose I can also just wait until the 12 months expire?.

4) Does a regular filing for a utility patent provide the same
protection (starting from the date it was filed) then a preliminary
protection? Meaning when I file for a utility patent, do I still need
to file for preliminary protection? What is the difference here?

I prefer a "predigested" answer to just a collection of links (where I
have to look up the answer myself). If you want you can provide links
but if your answer answers my question, I wont bother to look them up.

1.  A provisional patent application establishes an initial filing
date, which would give you priority over any applications filed after
that date that would infringe upon your invention.  However, until you
actually receive a patent, you cannot stop anyone from infringing upon
your invention.  You are, however, entitled to use the term "patent
pending," and, once your patent was issued, you could go after
infringers through the court system to try to stop them and receive
damages.  Your provisional patent application should be as detailed as
possible since your non-provisional patent application that follows
must be supported by it.  The provisional patent application is no
more or less vulnerable in principle to being "worked around" than the
actual patent.  The more details you provide about your invention in
the provisional patent application, the more difficult it is for
someone to file an application that would work around it.

2.  You will have 12 months, to the day (or the next business day, if
that day falls on a holiday or weekend), for filing your
non-provisional application after filing the provisional application. 
If you are, in fact, considering filing near the end of the term,
submitting your non-provisional application via Express Mail from the
United States Postal Service guarantees you a filing date of the day
you mail the application.  The same thing applies for your provisional
application.  You can potentially gain several days of precedence this
way versus regular mail.

3.  You are not required to file a non-provisional application. 
However, if you allow the provisional application to expire, you will
lose the precedence of its filing date.  Also, if your invention has
been "in use" or "on sale" during the provisional application period,
you may lose the ability to ever patent the invention.

4.  A non-provisional application for utility patent provides the same
protection in terms of precedence as a provisional application. 
However, a non-provisional application requires full details and
claims, whereas a provisional application does not.  There is no
reason to file a provisional application if you can produce a
non-provisional application by the same filing date.  The reason to
file a provisional application is that it allows you to pursue
commercialization of your invention for year with a degree of
protection for it without incurring the full expense of a
non-provisional application.

I hope you find the above information helpful.  I encourage you to
visit http://www.uspto.gov/web/offices/pac/provapp.htm for a full
discussion of provisional applications, a resource I used in
formulating my response.

Please request clarification if needed.

Sincerely,

Wonko

Perfect answer... I like people who can answer questions precisly.
Hard to come by these days.

Perhaps there is room to specify that the filing fee of a provisional
application is about 25% of a provisional one.

correction: ... 25% of a non-provisional one.