I think you mean styling the scroll bar, a trickery invented by MSIE which also only works with MSIE because it's not real CSS.
No, as it's not part of the standard, you can't validate it.
it's also not page styling but modification of the browser's GUI. Scroll bars are not part of your website, they are only part of the browser GUI, so there is no sane reason to manipulate them (that includes reinventing the wheel with flash scrollbars).
If he is going to put the navigation into a frame, I am going to put something hard into a place you don't want it to be.
Server side includes are the way to go if you are not generating the pages dynamically already.
The reason is that FRAMES are not meant to be abused for that. Some browsers don't support frames (yes, I know that that mostly concerns text browsers, but there are blind people out there too and blind people use text browsers).
In such browsers you will usually be prompted which of the frames you want to open. The very idea of putting the entire navigation into a seperate file is insane anyway (it also tends to screw up deep links, such as those presented by Google a lot).
You don't need a low bandwidth version. If you need one, you are doing something wrong. Ensure that your page works with Flash/Java/Stylesheets/images off, that's all. If people are on a slow connection and want fast content, they will disable images in their browsers, not to mention Flash. If they don't, they'll have enough problems going anywhere anyway.
Keep your markup clean and don't bloat it.
To get a general idea of what CSS and HTML are really all about (not the shit they tell you in internet related magazines), check out
http://www.csszengarden.com and take a look at a couple of the designs they let you chose from.
All designs use the same HTML and only different stylesheets and images (linked to via the stylesheets). While the HTML is a bit bloated with divs and spans and has way too many classes and IDs assigned, that was only done in order to allow the absolute maximum of customizability.