Well spotted! Great work scrutinizing this.

Having done a lot of video editing, here's my conclusion: It seems they tried doing a Warp Cut, i.e. collating two different shots and mesh Assange's position on camera. This doesn't mean evil intentions, they could simply be editing for time of pacing sake; not an uncommon practice in film. It would also explain overlapping of different collar positions, with a quick dissolve transition between the two shots.

However, this clip was definitely edited. Even though I don't see any clear evidence of chroma keying, I would not consider this footage to be legitimate proof of life.