Close a 7cm thick and 20cm deep groove in a load-bearing wall.

Oh dear. There is no way around the structural engineer.

Oh shit!!! If the structural engineer had you in trouble, you’re so thoroughly beaten that you also fit into the slot. Don’t ask laymen. Go to the structural engineer. There’s a reason why you are allowed to chase (vertically) a maximum of 30mm deep (depending on the wall thickness) in the load-bearing walls.

He said you could rebuild it nicely with swelling mortar and the removal of the stones.

Wonderful fantastic, the title of a fitting Duröhre video - I suspect by a fellow student of the OP - can be found with the search term "Stuffing swelling mortar over steel beams - livestream from the construction site". I have rarely been so thoroughly entertained. Really top notch, Mr. Kapellmeister. Or as it says around 24:10 from "Dannmallos - DIY": "but the result is simply fantastico".

I had spoken with a bricklayer. He said that one could also nicely rebuild it with swelling mortar and the demolition of the stones. I would like to put that up for discussion.

Successfully filling a hole does not mean that the structural properties of the wall are restored.

I don’t know what use a structural engineer is here. The ceiling should be supported using steel pipe props. The vertical slit is not a problem; even in load-bearing walls, cables and empty conduits run vertically that are as wide as the walls. However, the larger hole to the right of the wall I would properly remove so that whole bricks can be placed again. Preferably with higher density. So not patched up the way it looks at the moment. And your trusted bricklayer should actually be able to do that. In my company, it is routine to support load-bearing walls daily, install beams and lintels, and ensure stability. For larger openings, a structural engineer is needed. But in your case nothing is being changed, it is just being restored to its original state, and the load-bearing wall remains the load-bearing wall.

The vertical slot is not a problem, even in load-bearing walls cables and empty conduits run up that are as wide as the walls.

I don't see the structural engineer here either - but rather a really textbook joke of a DIYer, whom even grandma without glasses would see probably doesn't come from a practical profession. And to such an extent that I burst out laughing - although just a week ago I thought I already knew all East Frisian jokes.