The Many (Fictional) Universes

My attention has been refocused on alternative histories and parallel universes by:

reading Shadows Of Annihilation by SM Stirling;

watching His Dark Materials, which I might reread;

reflecting on -

Poul Anderson's Old Phoenix stories,
Neil Gaiman's Worlds' End,
the film, Last Action Hero.

As we have said before, any particular narrative may:

be set entirely within a single alternative history;
involve some knowledge of parallel realities, e.g., SM Stirling's The Peshawar Lancers;
involve travel between the parallels;
incorporate characters and events as fictional in one universe but real in another, e.g., a Lensman visits the Old Phoenix;
feature only the particular authors' characters or refer also to those of others, e.g., that Lensman, Sherlock Holmes and Huckleberry Finn in the Old Phoenix or Death from The Seventh Seal in Last Action Hero.

Does a single megamultiverse incorporate every possibility, including even scenarios explicitly denying a multiplicity of realities?

There will be more posts on this issue as well as on Anderson's Rogue Sword and his The People Of The Wind.