I don't think Explorer has that functionality.
Not that I know of; you have to use Explorer for that. Once rearranged in Explorer, Opus seems to respect the new order after an F5.
It's interesting that even in Explorer you can only rearrange the Quick Access items via the tree and not via the file display, which indicates it's something special they do in their tree control and not generic to the virtual folder. I'm not sure if there is a public API for other programs to rearrange the Quick Access list.