• For the log: Pharo.exe --headless <image_name> does nothing. Executed from windows command line in the directory where Pharo is installed (actually, extracted)
• NSSM
• full path to pharo exe
• working directory is the path where Pharo.exe is put
• parameter: name of production image pu in the same working dir
  
• enable interaction with desktop (haven't tested without itt
• all other options are default
• Service appears on service list. Started well manually.
  
• Seaside web server with installed app is alive and runs well
• Service manual stop function works (actually, kills it, no problem, this is a stateless server holding only volatile session data thus no shutdown callback is necessary)

That's all by far.

This seems to be a 100% solution to run Pharo WWW server on Windows as a service. Thanks for the many ideas and support.

Later on I may inform you on the results of real life situations (load, stability, etc).