Sound-insulated overflow elements for wall installation (controlled residential ventilation)

Schallex lets little to no air through at all. As a layman, I cannot imagine how air exchange can take place there. Or is air exchange in the utility room never needed in practice, since there is usually already the controlled residential ventilation installed there?

Well, you can also simply plan two valves in the utility room. It is practically cost-neutral since the piping is already there.

Are there good floor drain seals with overflow function? The classic Schall-Ex seals seem to me to always be completely airtight.

Yes, it exists. For example, Planet Min-E.