When I have a torrent with several files, some of which are set to "Ordered" priority (1...some number) and some others are at manual priorities, it seems like the ordered ones are higher than "Ultra Low" but lower than "Ultra High", which seems logical enough. What about other manual priorities? I believe that all the "ordered" stuff sits somewhere between two adjacent manual priorities, but where exactly? Between what and what?

If you're talking Aggressive/ON for First/Last piece and Progressive (streaming) for piece selection.. it doesn't CHANGE anything.
What it does is try to allow you to receive for that file the (HT is short for head-tail which is first and last piece of the file) information which will allow you to get info or stream it to check out the quality or "preview". For Progressive, there's a checkbox for the "issues" streaming can cause for the rest of the swarm. Tixati supports &ws in magnet: links as well as applying them inside the .torrent as "meta" status for HTTP based seeds. This is not webrtc, or wss: seeds which are newer technology.

There is no automated way to turn on progressive streaming. It's not "good" for the swarm.