Lamp installation help or ideas needed!

One can hardly estimate the distances. But a larger ladder leaned against the walls with the arrows should be enough.

No, unfortunately that won't work. The height could be managed, but the clearance for the lamp is already a good 2m-2.5m to the "back", so that the lamp doesn't hang in front of the window, but quite freely.

So exposing the holders doesn’t work anyway. They were chiseled into the stone and then filled back in afterwards.

At our place, they were just wallpapered over so they can still be used. That’s what they were specifically for, just like access openings in the bathroom.

But you don't have to have your feet at the level of the outlet and are probably at least 1.50m tall. If you have a platform, it's even easier. Then you wedge the ladder between the side walls. Or how high is the outlet above the platform?