"I could well be being stupid here." - not at all, unless we both are :)
there is often a confusion between flip and rotate, a flip will give a mirror image of the monitor, rotate spins it around the centre point.
you mentioned you had a method to flip (mirror) the monitor, you then only need to rotate the image so that when it is seen in the teleprompter mirror it is correct.
if you do not have a method to flip the display,
ultramon will provide this option, unfortunately it is not free but does provide a 30 day trial
https://www.realtimesoft.com/ultramon/
if the content is web based (and you have edge chromium) you could try the following google chrome extension (works in google chrome too)
open edge chromium, click the ellipsis(...) and select extensions
at the bottom of the left panel, turn on "allow extensions from other stores"
then get this extension from google
https://chrome.google.com/webstore/detail/mirro...
click the "add to chrome" button
it should then add to edge chromium
if you don't have the new edge chromium you can get it from
https://www.microsoft.com/en-us/edge
(it will replace the current version of edge)
hope that makes sense, please let me know how you get on
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