Glad to hear you are making progress. If you are getting artifacts around the edges of your key it could be due to video compression, almost any kind of compression can generate artifacts around hard edges, this could cause flickering at the edges of the key. In my stuff I’m working with uncompressed live video. If you are using compressed video, turning the quality all the way up might help reduce artifacts around the edges. I think they just added ProRes codecs to Unreal, but it might only be for output and not input of video.
Also something to think of, in your alpha channel video, what’s the RGB color in the area with transparent alpha? If it’s a strong color it will bleed through if there is any compression artifacts at the edges of your alpha channel. You might be able to reduce this by making the color of the transparent area something neutral (ie, not black, white or green) like a grey or whatever the predominant color of your 3d background is. So if there are any errors in the alpha channel the color bleeding through won’t be as noticeable.