Some of Hergé's architectural drawing was spot-on documentary realism, such as the centre of Brussels when Tintin returns home at the end of Land of the Soviets, or the airport, station and hotel in Geneva in The Calculus Affair.
And then much of his architectural drawing was carefully researched but altered, adapted, and relocated from their actual real locations for his fictional purposes. Examples of this would be Marlinspike Hall (adapted from a real French chateau, of course), the "Gestapo" house that mtc mentions above, the huge city-gate wall in the Blue Lotus (which I think I read somewhere was from a photo he had of a wall in a different Chinese city to Shanghai), and many "exotic" or typical buildings in Tintin's foreign adventures in South America, Asia, the Middle East, etc, which add a realistic flavour of the country.
And thirdly, some of his architectural drawing actually contains social and political satire, parody or comment. Reading what you've written in your first post above -
Herge did not just create scenes for his characters. The places he drew played an important role, as they depicted the local political and social circumstances.
- this category might be the most interesting to you.
The prime example of this category would be his creation of the entirely fictional city of Schôd in The Calculus Affair, where every building (or car) seem to have design features that refer to the country's dictator's huge Stalin-like moustache. (It's quite similar to the portrayal of Hitler's Germany in Disney's 1940s Donald Duck cartoon, Der Fuhrer's Face, where every road and window frame is swastika -shaped.)
Other examples of Hergé making socio-political comment with his architecture drawing would be the springing-up-overnight city in Tintin in America, and his portrayal of Tapiocapolis in Picaros, with chic skyscrapers and modern sculpture in the centre and terrible shanty towns on the fringes. And even that huge gated wall in the Blue Lotus, that I mentioned above, seems to have a political symbolism as well as architectural realism, somehow representing the occupying Japanese army's oppression of the Chinese people. (It's give an unusually big half-page panel, which adds to its impact as an image.)
Those are just my own thoughts really, rather than a bibliography of work on the subject, but I hope they're helpful! It's an interesting subject you raise, which is maybe not well covered in the few books we have on Herge's work that are published in English.
As to Hergé's own tastes and principles in architecture, I'm not sure. But I'd guess they were similar to his design principles on the page - utilitarian in a clean, elegant balanced way. (From photos, I think his own house was like that - fairly restrained and simple.)