Sheet feeders regularly produce documents in reverse, and merged files often need a page moved. Reordering is a drag and drop job, not a rescan.
A scanner that pulls from the bottom of a stack gives you the document backwards. The fix takes seconds once the pages are in front of you, and it is a great deal faster than feeding the paper through again.
Reordering and rotating in the same pass saves you a second round trip. A stack fed in at ninety degrees usually needs both, and doing them together means you only verify the result once.
The thumbnails render on your own device, which is why a long document takes a moment to display. Nothing is being uploaded during that wait.
Yes, drag it anywhere in the sequence.
Barely. The same pages are written back in a different order.
No.