Is a roof window really not feasible with a roof pitch of about 12°?

Look rather for flat roof windows or skylights. They also come with a motor for opening. They are designed for 0-15 degrees. For all I care, also from Velux.

The site manager said that the installation of a flat roof window is not possible due to the necessary weatherproof underroof - I did not understand why this should be the case.

What kind of roofing are you getting? Waterproof underlayment sounds like roof tiles/slates. Maybe that's the reason? Have you been explained why something like that would work on flat roofs but not on low-pitched roofs? I would first have that technically explained by the architect.

The roof covering will come from Braas; the HARZER PFANNE F+ will be used, which for the first time allows the covering of a shallow pitched roof from 7° with Braas roof tiles.

The architect has so far argued that Braas would refuse the warranty when installing a roof window.

Therefore, I have now inquired directly at Braas where I was told the following: "As long as our Braas products are not altered/affected and our installation guidelines are followed, the warranty/guarantee is granted."

All of this seems very strange to me.

Well, that's because Braas m are outside the recognized rules of technology with their system. The standard roof pitch for roof tiles is, as far as I know, 22 degrees, up to 16 degrees with a waterproof underroof. You need this, among other things, because due to the low roof pitch, rainwater can be pushed under the tiles by wind. The flatter it is, the worse. From 22 degrees onwards, this apparently no longer happens, so a rainproof roof is no longer necessary. With Braas 7F it is now the case that Braas has come up with something so that they do not have any problems even at up to 7 degrees, provided you use their (tested) sealing membranes for the underroof and do not just say "make some kind of underroof". Of course, it may be that Braas says, "no, warranty only if closed roof surface, otherwise no guarantee." Then a window in that area is naturally difficult. But the exact details must be given by the executing company, because it is they who will later take over the warranty towards you. That’s my layman’s opinion as a builder who chose the same roofing, only without a roof window.

Hm, the point is precisely that Braas, knowing full well that we have a roof with the mentioned pitch and will cover it with the 7-degree tiles, has no problem with the warranty:

"As long as our Braas products are not altered/affected or our installation instructions are followed, the warranty/guarantee is given."