is there a diference between deleting objects by backspace or delete button?

No.

i have modeled a cube with a y-dimensional of 100 units.
the top view doesn´t show the top of the model. also other objects get invisible.
i know that objects, which are placed in a far distance disappear in the viwport view until you raise up the far clip plane value of the camera.
but what do in my case?

I don't understand the question.