I really can't understand that. Install the windows first, then the interior work can be done. What's the problem with that?
Windows are installed "after" the brick cladding, not before. Otherwise, no warranty is provided for the installation. At least that's how it is handled in our region.
I don’t quite understand why you’re doing the egg dance with some field staff, intermediaries, and manufacturers. Your contracting partner is someone else, isn’t it?
100€ for the shell builder to close the affected openings. Then the interior work begins. Although I really don't see the problem with installing the window beforehand. Otherwise, I advise taking the necessary steps against the contracting party. The production manager of the manufacturer is completely the wrong contact.