Navarro/Bommentre earned their top 75 PB at Karl Schaefer last year. (They had the 38th best score from last season.) Every other couple who is competing in Seniors this year whose PB was above N/B got two invitations as well.
The explicit advantage that host nations get is in the discretionary invitations. Without them, only Wang/Meng at CoC and the "TBA" at NHK (replaced by Hajkova/Vincour) wouldn't have been issued invitations.
I've edited your post to quote, but thanks very much for explaining all the permutations of qualification for the GPs - it's very informative indeed, and less of a *cartel* than I'd previously thought.