According to a calculator on the internet, for the quantity of 24 tons of crushed stone (compacted), approximately 13m³ of excavation should have been done beforehand. This should include the crane and the driveway, unless he did not charge you for the crushed stone under the crane. This would mean that the excavation for the extension has increased from 34 to around 80m³, and I would at least have that explained by the structural builder. And as has already been written elsewhere here, according to VOB you have the right to demand a recalculation if there is an excess quantity of >10%. It is important that you insist on this, as the structural builder is not obliged to do it on his own initiative.
Regarding the architect: You have no right to be personally supervised by him. Nor would you have the right with the craftsman that the master, and not his journeyman or even apprentice, works for you. The former architect is liable for the errors of his employee, and that must be enough for you (as unsatisfactory as that may be).