I thought that understanding neutrons, was a done deal per Standard Theory?

Parallel universes is unlikely for empirical verification. Part of the multiverse bollocks in QFT interpretation ...

For example we have a Taylor expansion of some fancy function. It has an infinity of terms that hopefully converge.

Is each term in the Taylor expansion, a separate "law" in a separate universe (C0 universe, C1 universe etc), with the sum of the infinite series ...

a "path integral"? Isn't this a problem of "Pythagoran Realism"? The notion that maths not only is a useful ...

quantitive model for physical processes, but that maths is what it actually is?

If you have multiple universes that have the same laws in each universe, you simply have duplication. If you have separate laws in separate universes, then if they are really separate, then again the other universes don't matter. Only if they laws are different, and the universes aren't really separate, do you have an interesting claim.

But if the multiple universes interact, and they have different laws, how can scientific method proceed? Different space-time zones will have different phenomena depending on how the different universes interact differently. This violates the fundamental notion of uniformitarianism (laws are for all space/all time).

... sequence, series, convergence, divergence ...

... adding a variable to a converging series (converges for some values of X). Generalizing geometric series ...

Famously, a fancy function can be represented by a trigonometric series (cosines, sines), not just a power series (Taylor's Series is how we can define cosine, sine etc).

Solutions to QFT equations famously converge slowly (even after renormalization). So it is hard to calculate. Now imagine a graphical way of doing this ... Feynman diagrams. Each Feynman diagram represents one term in a slowly converging series. "Path Integral" is is simply an alternative way to add up the terms. In this case, we are seeing a single universe that is calculated by adding up an infinity of terms that don't have their own reality.

But if the terms don't interact linearly (just simple addition) we have a new problem. This is like having multiple universes (each term) interacting with each other. An example of violating uniformitarianism ... wormholes. A wormhole within one universe is interesting. That means the topology (knots) of space-time vary from place/time to place/time. Interacting multiple universes would be like ... a wormhole between two different universes. But then are they different universes? Only if they laws on one side are different on the other.