Best glide speed vs. best glide angle? Maximum glide range?
Is it true, that this best glide speed is given in terms of indicated airspeed IAS and hence is independent of the elevation?


Yes. The best glide speed needs you to fly at the optimum polar point, and with the same IAS you will fly at the same lift coefficient and polar point.


  To determine the glide path angle I suppose, that the ground speed and the sink rate is relevant.


No, the angle will be the same regardless of altitude (if we neglect Reynolds number effects for now). What does change is the speed over ground - this goes up with height.


  What is the resulting flight path angle when maintaining best glide speed? Does it change with decreasing elevation?


No, it doesn't. The glide path will be the same, only at higher altitude the airplane will fly faster and proportionally sink faster. What changes is the time scale, not the geometric scale.

EDIT:
The added graph is either given in TAS on the x scale and for a specific altitude, or it is given in IAS and valid for sea level to maximum ceiling (if we neglect Reynolds number effects for the moment)

P.S.: Terminology:

Altitude is the vertical distance from sea level to where an airplane flies. See here for more.

Elevation is the vertical distance from sea level to the ground or to an obstacle.

Height is vertical distance between an aircraft and the local ground.