If it now appears that this video was faked in at least one significant way (McGrath)

Faked? I'm not sure. I have not seen the video, only read a transcript of the words together with a description of the action. What I have read left open whether Berg was alive at the time of the decapitation or not. The video seems to be edited by the distributors, that's exactly what you would expect from a propaganda video. And that it is: a propaganda video for jihadistas.

In any at least a tiny bit complex happening, the reports will sometimes contradict each other, and not all the details will be completely clear. To infer from that that the whole story is fabricated is argumentum ad ignorantiam.

Governments have lied and governments have told the truth. So, you'll find innumerous instances for both. The shortcut 'they have lied to me more than once, so they lie here too' is attractive for lazy thinkers but ir may turn out not to be a shortcut but a mental shortcircuit.

The Secret Service in Macedonia has conspired and killed Pakistani two years ago. Does this instance make it any more likely that some US personell has killed Berg? No. That example is only relevant for those who believe that these things have never happened, but I see nobody here with such an opinion. It also would be completely irrelevant if I told an example in which a conspiracy theory turned out to be wrong after all.

You'll have to judge this story here on its own merit. The tape seems to be edited, perhaps the video shows scenes from different cameras. That action by itself is easily consisted with the murderers editing their tape for propaganda purposes. Apart from that, I have not yet seen one reasonable argument for the murder being a 'black operation' by US psychological warfare specialists

Wolfgang