I'm wondering if this thread is on the correct forum at this point - I was about to make a snarky remark until I recalled that we had set up a separate forum for addressing certain topics, and I think you might be veering in that direction. So I'm forced to keep my snarky comment to myself for the time being.
Having said that, does the theory hinge solely on the tunnel, or does anything else support suspecting the bishop at this point? What about the shaft of light? And the bad guys here, given your theory is correct, are operating in a particularly devious and cunning manner, and employing an extremely light, deft touch with Sir Ewen & Co. As the GM has pointed out, this is characteristic of an entirely
non-Morgathian approach, but a recognizable approach nevertheless.
Which brings me back to my snarky remark. But perhaps now you can guess what it was