It seems that you managed to solve all but one of your problems.
Do you want you custom styles to be available only inside the editor (authoring mode), or also when you view the list (presentation mode)? The SharePoint server uses our editor in authoring mode, so we are able to control the Css styles on the page - you can open the ListConfigFile.xml and add the CssFiles property there:<property name="CssFiles">
However, as I already said, this will only change the Apply Class dropdown list in the editor (authoring mode).
If you want to add a Css file in presentation mode, you need to do it in some alternate way. MOSS does not load our editor control in presentation mode so we have no control over what is rendered on the page. If you want to customize the styles, you need to modify the page template itself using SharePoint Designer or add the styles to one of the css files that are already loaded. Core.css is such a file (it should be present in all pages) - \Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\STYLES\Core.css
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