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So, there are some great tutorials around the web about isolating render passes in Cycles so that you can make changes, and then recombine all of your work as one complete image.

You'll probably have a Compositor node setup that looks something like this:

Separating render passes with the Compositor

Blender's Manual used to have a schematic explaining how these can be recombined:

Cycles render pass combination (deprecated)

But not all of the pass types are explained by this deprecated schematic, as now there are more pass types (SSS, Shadow...)

What would be the "master" routing, so that if all passes are separated out they would all get recombined properly to form the complete image?

I set off on a mission to create such a routing, but realized I wasn't sure how to recombine some of the passes.

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SSS works just like the 3 other "light type" sets do, so you add SSS direct and SSS indirect, multiply the result by SSS color, and sum that result with the other sets.

Shadow is not an image component per-se, since if you rendered your lamps with shadows enabled, they will already be embedded in your lighting passes. (the shadow pass already existed at the time that graphic was created, btw).

So to answer "what is the master routing?", it's just that with the SSS set tacked on the bottom, working the same way.

There are currently no volume passes, so volume shaders cannot be re-assembled in this way. What you can do, however, is use a handful of passes subtractively. For example, let's say you just need to edit glossy indirect to brighten reflections and clear some fireflies, you don't care about any of the other passes.

Rather than render EVERY pass and recombine them all, you can just render the passes you need, subtract the unaltered from the master, edit it, and add it back. For the light passes, you need to do the add+multiply to get the full lighting pass, rather than the split raw passes: example network of subtractive passes

This will work with any shaders, including volumes.

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