The thermostat sounds very resonable, I will change it sooner or later. Otherwise the coolant circulation looks like and feels like normal.
I removed the duct tape. Here is a photo: http://www.rantakallio.fi/images/heater2.jpg, also the lever is positioned on the opening, compare to the first picture.
The opening is factory made, it is obvious. Nothing seems to be broken, something is just missing, but what...?