Is it possible to use the Arduino for industrial use?
Yes
Is it reliable enough. Is it built to run 12 hours a day?
Yes
Is it also possible to built the Arduino in a product which is mass-produced?
Yes - but:
Which possibilities do I have to turn the Arduino in a suitable device?
More likely to be cost beneficial to build your own PCB with IO, etc, that meets your requirements.
For example, use a wallwart 5V power supply, delete the 7805 regulator, add IO connectors that are more secure than plug-in headers, etc.