Hello everyone, the appointment with the expert was last Friday. The owner had all the documents and photos, construction plans, etc. there and he looked at them and knew every single company that built it back then and said those were (and still are) the best in the region. He examined the masonry, etc., and found nothing that gave him any cause for concern. The exact construction plans (and photos from the construction) revealed that the peat lens was not very deep. The house was founded on a total of 34 concrete piles, each 1.2m wide (well foundation). He said that is also absolutely common in the region.
He looked at the pool; the water and the mud come only through this opening for the pool lighting, not through the base slab. He suggested hiring a mason to seal the holes with a few stones and then fill them with concrete. Afterwards, the pit could be cleaned and then used as desired. The walls in the room are all dry. In conclusion, he assessed that the house is in excellent condition for its age.