Does Mixer support Xbox360 backwards compatibility broadcasting on XB1?

Yes it does.

I've streamed multiple backwards compatible games using Mixer

This should also work for the original Xbox games that will be backwards compatible end of the year into next

The Kinect is not supported for a list of supported webcams please visit: https://support.xbox.com/mixer/broadcasting/broadcast-with-webcam-on-mixer

Hello Cameron,

Thank you for posting on the Xbox forums. We understand that the aspect ratio of the game is changing when you start broadcasting on Mixer. There may be some

broadcast and capture settings that need to be changed before the game can be set to full screen. If changing the broadcast and capture settings does not resolve the issue, we suggest reaching out to the
Mixer support team. Be sure to let us know the results!

Guide to using Webcam with Mixer

You can use a Webcam with the Xbox as long as it's compatible, not sure on the specifics of twitch, but I'm sure some other folks out there can answer that..

Is the disc for a backwards compatible xbox360 game as only discs for backwards compatible games will register.