You asked a lot of different things, so I'll just try to focus on one:
My main query is this: If every possible world exists, and quantum probability simply emerges from the abundance or perhaps density of worlds according to the properties of the wave function/Schrödinger's equation, then there are still real worlds in which very unlikely events happen. For example, there is a world where every Stern-Gerlach experiment reads every spin of every particle in the same direction.
Yes. If you like to think of it as "quantum coin flips", there's a very small but non zero chance of ending up in a world where the quantum coin flipped 100, 1000, 100000 heads in a row, so to speak. And if many worlds is correct, there is someone experiencing that truth.
And, in a vanishingly small number of worlds, the quantum coin flips luck is so extreme that quantum mechanics itself is not as obvious as it is in most other worlds.
