Exterior walls made of aerated concrete, interior walls made of calcium silicate brick - yes or no?

If the structural engineer says so, it must be true. For us, (as of today) there will be a maximum of 175mm and only a little of that, but we also have KS on the outside and no aerated concrete. The architect already said, however, that if the structural engineer is not satisfied, you first increase the raw density - there are different variants for KS - and only then increase the width.

As a load-bearing wall, we have the 24 cm KS in the middle of the house. [...] As a layman, I assumed that this was correct

And it is. Depending on how many load-bearing walls there are (and at what distance from each other) sharing their loads, a 24 cm wall is quite appropriate. If planners generally place 17.5 cm walls everywhere except for the WC, pantry, cloakroom niche, and shower partition wall, it naturally looks different. However, that also means fewer walls are "removable."

Cracking* is not to be expected... ask him if he will give you that in writing.

*There will be cracks, regardless of the material and even if it is the same material everywhere. Guaranteed.

Thank you all! We will now have [KS] installed inside.

what do you use 24 cm KS inner walls for? On special request? With 24 cm KS you build basement exterior walls

So our basement exterior wall is 30. But I agree with you, 17.5 would normally be enough for load-bearing walls...

17.5 would normally be sufficient for load-bearing walls...

24s are not chosen out of whim or excessive caution. As a support under a continuous ceiling, I can well imagine a 17.5 being sufficient, but as a bearing for two adjacent ceiling panels there, not really.

And, as mentioned, designing (almost) all interior walls as load-bearing has, like all coins, two sides – then you simply can't remove any more (without putting another one in their place).