Boiler does not fit in the niche. What options are there?

Or revive the original plan?

Unfortunately, there is no chance of that anymore.
Originally, the heater was planned for the opposite corner.
However, that was not practical, as the house connection was introduced from the street in that corner, and the shortest path to the outdoor unit is where the heating system is located now.

And the walls in question are all load-bearing, so you can’t steal anything from the wardrobe or pantry or whatever is beyond the niche?

Unfortunately not really. On the other side of the wall you mentioned, a kitchen unit is flush installed. The wall is not load-bearing, but it is only 11cm thick. The plumber will certainly need about exactly this measurement. It also involves 4 connections that need to be installed. The right wall is an exterior wall. I don't think that's very good either?

The right wall is an exterior wall. I don't really think that's a good idea either?

But at least you can access the connections well from the outside