Why do you think, that this is Citaro LÜ, not usual Citaro L?

Because all the seats are on a platform, like in the Citaro Ü or GÜ or like in the MAN NÜ.

Well inside of the bus can be made any kind. Maybe there is a chance to take photo of it's VIN plate to see if it's really Ü. I think that Ü have narrow front door (only one half, not two).

The doors have nothing to do with the model. A normal Citaro G can also be ordered with a narrow front door (some of these Citaro G's are riding in Amsterdam). A Citaro Ü, GÜ or LE Ü... has also the fuel cap for the diesel on an other place.