There are two High Earth/Traitors' Guild/Green Exarchy timelines, one in The Quincunx Of Time with the Dirac communicator, another in "A Style In Treason" without it. A teacher in a school where I worked suggested this model exam question:
"Difficult things have simple explanations. Discuss."
We read James Blish's works because they are imaginative hard science fiction with human characters responding to fantastic situations in alternative futures:
a journey through the microcosm, including landing on the surface of an electron;
messages from the future;
the end of this universe;
encounters with intelligent energy beings;
judgement by an older, more powerful race;
adaptation to microscopic aquatic life;
expansion through the galaxy in many adapted forms;
the demon fortress, Dis, appearing in Death Valley after a nuclear war.
In one very important sense, this is realistic fiction. If the human race survives, then it will encounter many fantastic situations. (Sf writers, including Blish, also present scenarios where we do not survive.) When anything that has been imagined, like space travel, does happen, it usually happens very differently from the way it was imagined. However, any speculative writer's resources comprise only current scientific knowledge and his own creative imagination. This is enough to generate masterpieces like Cities In Flight, The Seedling Stars, After Such Knowledge and the three diptychs.
With the Haertel Scholium, comprising After Such Knowledge Volume III, the diptychs and a few short stories, we can get tied up trying to fit together cross-referencing novels and stories that do not form a linear sequence but this should not detract from appreciating individually high quality works.