The engine control unit is a little different as far as what can be changed. Much of the info is "burned in" I guess you would call it, and can't be changed. This is part of the anti-theft features of the car, very complex on newer cars like this. You can go in for example and look at the VIN number, but not change it. There are only a few changes that can be made, for example on the new V12 I know you can turn the cylinder shut off feature on and off. You can also shut off individual fuel injectors and coils as diagnostic tests. Operations can be performed electronically such as a compression test, using the crank angle sensor/flywheel speed. There is so much info there it's hard to believe, which is why it would be easier to discuss if I and the techs here knew what you actually want to know.
In some control units, certain changes will require a password from MB, yes. I don't recall having to use this on an engine control unit though.
MB is just in the past 6 months or more starting to make the control units flash-updateable. In the past, any computer upgrades or corrections were made via a control module replacement.
I am unsure how to describe the OS or programming language. In the example of the ECU, I believe it would be Bosch proprietary (Bosch ME engine management system).