So if you are doing a strip always do it larger, and it will look better printed at 70% size as a result. Obvious point really as standard practise with cartooning!
You're right that many cartoonists and illustrators - including Hergé - draw or drew their artwork bigger than printed size, but by no means all do. Many illustrators and cartoonists I know (including myself), and many others whose originals I've seen, prefer to work at same size. Whilst you're right that reducing artwork when printing tightens up the look of a drawing, this isn't always a good thing. What you gain in tightness, you can lose in immediacy and warmth; reducing a drawing can distance it from the feel of an original drawing, as well as making the finer lines illegible. Clearly that isn't the case with Hergé's work, but I guess he got pretty used to knowing what the drawings would look like when reduced to his standard printed size. Personally, I think he'd turn in his grave if he could see the ultra-shrunk versions in today's 3-in-1 volumes!
So I don't think there really is a standard practice. In the book illustration world, I'd say that same-size illustrators are in the majority. But maybe in the comic magazine industry, reducing is the more common approach. As a same-sizer, I've noticed that many people who are interested in illustration but don't draw themselves assume that all
illustration work, especially pen-and-ink work, is drawn bigger than printed, and they tend to take a huge amount of convincing that anything involving any detail is drawn same size. People often underestimate how fine a line you can get from a dip pen.
I don't know how he got those wonderful straight architectural lines, he must have used a ruler.
My guess would be that he certainly used a ruler for the pencil lines, but probably penned these straight lines freehand, to avoid deadness.
In The Blue Lotus album he appears to use a thin nib without an edge, as the line seems consistently thin. Then he changed to his normal pen in The Black Island etc.
I think the thickening of his line is a more gradual and inconsistent process than that. The Blue Lotus line stands out as particularly thin coming after the preceeding books, Congo, America and Cigars, all of which are 40s and 50s redraws, of course. And his line is still quite thin in the books following The Blue Lotus: The Broken Ear, the original Black Island, King Ottokar, etc, gradually tending to become thicker and lusher throughout the 40s, 50s and 60s.
The 60s redraw of the Black Island has a particularly thick line, which according the Harry Thompson's biography of Hergé is because the artwork for that book was drawn closer to printed book size than other 1960s artwork, so the line width is less reduced. Conversely, maybe the thinner looking 1930s line is due to a greater scale of reduction between artwork and printing, as well as a thinner pen nib as you speculate. I'm not sure; we'd need to know if his 30s artwork was generally drawn bigger than his 1950s artwork to know if that was a factor.
I also wonder how much factors such as stress, rush, experimenting with new nibs, or which of his many assistants were working on the background penning affect the pen line from one book to another. To me, the pen line in parts of The Red Seas Sharks looks a bit different from that of the preceding book The Calculus Affair, for instance.