Hello.

Please find my answers below.

Ad 1)
Do you mean Description field on Product/General tab, when being in CMSDesk -> Content -> Edit mode over some product? If so, there is already FCKEditor used, however with 6 options only (Bold, Italic,'OrderedList, UnorderedList, InsertLink, Unlink). This definition is based on definition of Basic toolbar, as you can find it in \CMSAdminControls\FCKeditor\fckconfig.js file. By default, is contains these 6 buttons only, however you can extend it by any other supported buttons. To have full toolbar, you can use following definition:

FCKConfig.ToolbarSets["Basic"] = [
	['Source', '-', 'Preview'],
	['Cut', 'Copy', 'Paste', 'PasteText', 'PasteWord', '-', 'Print'],
	['Undo', 'Redo', '-', 'Find', 'Replace', '-', 'SelectAll', 'RemoveFormat'],
	['Bold', 'Italic', 'Underline', 'StrikeThrough', '-', 'Subscript', 'Superscript'],
	['OrderedList', 'UnorderedList', '-', 'Outdent', 'Indent'/*, 'Blockquote'*/],
	['JustifyLeft', 'JustifyCenter', 'JustifyRight', 'JustifyFull'],
	['InsertLink', 'Unlink', 'Anchor'],
	['InsertImageOrMedia', 'QuicklyInsertImage', 'Table', 'Rule', 'Smiley', 'SpecialChar', 'PageBreak'],
	['InsertBizForms', '-', 'InsertInlineControls', '-', 'InsertPolls', '-', 'InsertRating', '-', 'InsertYouTubeVideo'],
	'',
	'/',
	['Style', 'FontFormat', 'FontName', 'FontSize'],
	['TextColor', 'BGColor'],
	//['AutoSave'],
	['FitWindow', 'ShowBlocks']		// No comma for the last row.
];

Please note toolbar name must be Basic, as it is used there. You can find more info about this topic here: http://devnet.kentico.com/docs/devguide/defining_custom_toolbars.htm

Ad 2) You can add additional fields into COM_SKU system table, which can be found in Site Manager -> Development -> System tables section.

Ad 3) Yes, this behavior is by design. The main reason is that one product can be used on multiple documents, so update in this product -> document way is not wanted.

Best Regards,
Radek Macalik