Sealing floor-level windows

Windows were sealed during installation except at the bottom, bitumen was applied after window installation but was omitted in the window areas about 60cm high

You are mistaken about that - sealing is done beforehand and then again at the connections after window installation.

Yes, you are right. Seal the shell once, then the window. The latter is typically done by the window installer, possibly?

No, who typically does that, I already wrote here ;).

The roofer

I think now we are bringing in too many things, previously my entire ground floor was completely sealed with bitumen waterproofing membrane, the only thing that was overlooked in my case is the sealing from the outside below the windows, tomorrow the exterior plasterer is coming to take a look at it after all he is partly to blame, it should have been noticed.

No, he can't do anything about that at all...

Doesn't the VOB say something like every trade must check the previous trade beforehand? or something similar