What is the least noticable way to re-stream a video to Twitch?
You should try FFsplit, is easy to use and free. With proper settings you should loose few or no data at all.


Setup the encoder to match the quality (bitrate and aspect ratio) of the original file.
Add a new layer of window capturing, and adjust width, height and origin (x,y) to capture only the region of the video
Setup hotkeys on ffsplit so you can start the streaming when the video player is on focus


Start your stream first then play you video.

Find a quick-start guide here on twitch.tv help, althoug this an old document, the new version of ffplist has few, but nice changes