It's obviously the same guy, the conspiracy vid is pathetic. People look different without hair + the comparison video they use was taken shortly after the last time he was kidnapped when he looked quite thin. He was captive in Syria for a while and they obviously fed him fairly well, he wasn't going to get much exercise while kidnapped either, so why wouldn't he put on weight?
He was a well known journalist. If none of the many other respected journalists who knew him have questioned the likeness, or indeed any of his family, it's unlikely to be a fake. Unless they are all in on it, but if that was the case, "they" must already be secretly running the worlds media and governments, in that scenario, what purpose would a fake beheading video serve?