Always so much griping on MFL threads!
OP, get your DC to look at the course detail for each institution, not just the Uni. They all vary, but these days there is so much more flexibility to tailor modules to a student’s interests/career aspirations. Whether taking a single, joint or multi disciplinary honours route.
Re year abroad. Again, whether 1,2 or more languages taken or mix with another course, there can be options to split year abroad, focus on one (but go to an overseas uni that still has classes in the other, or, take advantage of summer programmes.
Re covert native speakers, or those that have declared their native connections, whilst this is one of the reasons for language take up decline (on top of accessibility across some UK school sectors), the upside is that being in class with natives can improve non native language skills. Also, whilst they may have strong speaking skills, grammar may not be as strong. Language is not the only component of an UG degree, and some students with native heritage may have career aspirations in their native homelands (even if not born/have first hand experiences etc) so I would not discount their motives completely. But yes, some take for an easy degree too.
Finally either way, taking history combined with German or language with history integrated in programme, DC will get exposure to both faculties, and all the benefits that come with widening their network and peer groups, whilst covering their personal academic interests.
Best of luck on shortlisting options!