Let me clarify one thing about this issue. As you mentioned, there was B Final in 4x400 m where Czech Republic finished 3rd, total ranking 11th. Thanks to DQ of US team they moved up to 10th place and directly qualified for WCH. This is what I am happy about, proud of.
But one thing about qualification from WCH in Doha to SOG Tokyo. In the qualifying standarts is nowhere written that "ALL FINALISTS" will automatically qualify. I think, this is a small misunderstanding that IOC or IAAF should correct a bit. If a team in Final A has DQ or DNF, it means that the team has no final results within the championships. It has qualifying time/place to the final but in the final itself could be for the team either DQ or DNF. In this case, if somebody has really no result in the final, the 8TH DIRECTLY QUALIFIED TEAM for Tokyo 2020 should be the team, which is basically 9th in the final ranking - 1st team that did not make it to the Final A.
To clarify, I am not saying how is it gonna be. This is just my common sense point of view how I think it might happen. In my opinion it's the most logical way.