Typical litigation costs to enforce a patent through trial are one to one and a half million dollars on each side.  It is common, however, to enforce a patent without going all the way to trial.  It may cost only a few thousand dollars for an attorney to contact suspected infringers, and either get them to either stop producing and selling the infringing product, or take a license.  Of course, there are innumerable variations in between.

No.  Patents are not self-enforcing.  It is not a crime to infringe a patent, so the police, district attorneys, and so forth will have no interest in helping you enforce your patent.

Should I go through a patent marketing company?

No.  Although there may be exceptions, every patent marketing company that I know of does a horrible job of both securing a patent and marketing the technology.

No.  Sending out your own nasty-gram will only tip off the suspected infringer that you have no money to pursue an infringement claim.  Hire an attorney.

There are companies that sell offensive patent insurance, which pays about 80% of the cost of going after an infringer. However, such insurance polices are difficult to get.  Obviously the insurer will obtain its own opinion regarding the likelihood of infringement, and will not write the policy if it appears that infringement is likely.  The policy certainly will not cover instances in which you know about the infringment before purchasing the policy.  The cost of such policies is about 5% each year.  Thus, for a policy with a one million dollar  limit, the annual cost would be about $50,000 per year.

It is possible for a patent holder to obtain a “reasonable royalty” for activity of a competitor in the window between publication of the patent application and the date of the patent issuance.  Unfortunately, the royalty is only available if (a) the issued claims are substantially the same as the published claims, and (b) the competitor had “actual” knowledge of the published application.

The short answer is yes.  EPO Article 67 provides for reasonable compensation between (a) the time of publication or notice of the granting of the EPO application, and (b) actual issuance of the patent in a given country.  However, the laws vary on provisional protection from country to country, and in most countries, the provisional protection begins only upon publication of the claims in the national language.  The EPO has a web page with links to the laws of each country .  In any event, the remedy is limited to damages.  A patent holder cannot use provisional protection to secure an injunction that prohibits a competitor from continuing to infringe.