Hi Keith,
You’re welcome. Please contact me privately if I can help in any way. As you may have gathered by now, I am a style sheet geek and power user. 
For brainstorming and designing Scrivener style update features, I recommend also finding a machine that can run Classic and exploring the style sheet implementation in WriteNow 4 (or 3). WriteNow 3 was the earliest word processing program I know of to implement both paragraph styles and character styles. And they did it WELL. Not perfect, not as powerful as FrameMaker’s implementation, but incredibly well thought out and easy to learn.
One of my favorite features of WriteNow styles is that you can put the cursor at the beginning of the first instance of a style in a document, hold down Option, and any format changes you make only apply to text whose style matches the beginning of your selection. This makes it easy to change all boldfaced text to another font, and give it a unique color, for instance.
Another of my favorite WriteNow features is style dependency. This is an early version of cascading styles that long predates CSS.
Let’s start a WriteNow document with two styles for body text, Body and Header. You set Body as 12-point Galliard flush left (a.k.a. ragged right), then create some dependent styles, Body-Indent and Blockquote, with different ruler settings. Checkboxes let the dependent styles inherit various features from their parent style. So for instance if they inherit font but not font size, if you change Body to 14-point Georgia justified, all instances of Body-Indent and Blockquote will also change to Georgia, but stay 12-point and keep their ruler settings and alignment.
If you set up Body-Indent, Blockquote, Bulleted-1, Bulleted-2, Footnotes to all inherit their font from Body, changing Body’s font will update all the body text in the document – but not the heads and subheads.
Likewise you can set Header to Arial 20-point centered, then create dependent styles Subhead A (inherits font and size from Header, but is flush left and has space above) and Subhead B (inherits font, alignment, and spacing from Subhead A, but is 16-pt). Now if you change the font of Header to Papyrus, Header will be 20-point centered Papyrus, Subhead A will become flush left 20-point Papyrus, and Subhead B will become flush left 16-point Papyrus. But the body text will remain unchanged.
It would also be worth using FrameMaker’s style features, or having someone demo them for you. FrameMaker originated on UNIX, so the interface is extremely powerful but somewhat user-hostile. I like its features so much that it was my writing tool of choice for many years, until Scrivener came along.
My favorite feature of FrameMaker styles, other than the Book File updating I described earlier, is that you can define a set of style parameters, such as font and font size, then search and replace them – not just in a document, but in ALL of a Book File’s included documents sequentially. This allows easy style conversions when importing from another app.
Another cool feature is that if you apply unique formatting changes in FrameMaker without creating a style, such as turning a particular word green, when you go to update document or Book styles, FrameMaker will warn you that there are instances of non-standard styles, and give you the choice to keep them or override them when you update.
BTW I have only used FrameMaker on Classic. I was told several years ago that the new Linux version will run under OS X. (Adobe stopped supporting the Mac version when they bought Frame.
)
Most people these days have only been exposed to badly-done style sheets (Microsoft Word, whose style sheet implementation is a bug-ridden abomination) and fake “styles” like Cocoa’s that are only style presets, and don’t update. Word in particular has given styles a really bad name.
Which is too bad, because style sheets are one of the most powerful tools out there, and a huge time-saver and hassle-reducer for writers – as countless people who maintain websites and tweak their style sheets can attest.
HTH
All the best,
Joy