I can't scrub the Movie set as image texture in cycles I want to do Rotoscoping, in the way you do for animation where real life footage is use to inspire the animation of a character.

I almost got it working according to these instructions: Movie set as image texture not playing in cycles?

Except I have to hit in Mapping the Texture button to view a changes of the video texture, that kinda takes care of the problem except I can't scrub the animation which makes it difficult to animate this way.

Maybe it's the MP4 maybe it's the 2.79 version that is buggy I need help trying to figure out why it is not working for me.
Do you need the textured plane to rotoscope from? The background image works fine if you change the input button from "Image" to "Movie Clip", and deselect "Camera Clip". The movie (that I used - yours wasn't attached) worked fine for both scrubbing and playing. The plane would update OK after I stopped playing the video and would snap to match the background image. Oddly, if the properties panel in the node editor was hidden, or if another node was selected (other than the texture node), the texture stopped updating on the plane. This looks like a bug to me that you might like to report (from the menu choose Help --> report a bug). Good luck.