Upon pasting an Excel (selected portion) of a sheet, the grid or cell borders do not display.

That's the idea, that all styling is cleared. You should style your table through CSS. It means that any table you'll paste will be automatically converted to your website's styles.

That makes sense to have a CSS for the widget. This brings about another question. How can an author of an article select from multiple CSS for a widget? Example: the Table widget contains two CSS styles: default and fills. How is the author to select one or the other?

You have that power. You can have predefined styles that user can choose from. Defaults are "Caption", "Signature" and "Note". Click "Insert/Modify table" button in "Table" widget and see the "Class" option.

If you add "My different table" style, user will be able to choose that style from table properties. When user chooses different class it's up to you how to style it.

Here's a "how to" to do that - http://docs.impresspages.org/en/left/ho ... xt-editor/