I just finished listening to the audio book version of Peace Talks after having read it about 10-11 days ago. I didn't noticed the first time through that Harry gave Alfred instructions that Thomas can only speak to another prisoner if that prisoner is undergoing the same punishment, and there's only one other prisoner whose doing the contemplation thing.
The alternative is that after talking to Thomas the British prisoner will speak to Harry. However it plays out, what do you think will be learned from all this. I have a feeling it will be about a cure for Thomas that involves Excalibur. There might also be some information about how to breach that special bronze armor the Titan's wearing.
I agree with your reasoning. I think Thomas is being set up to wield Amoracchius.
The imprisonment of him on Demonreach is going to be an evolution of his character, into someone really worthy of taking up the Sword. Amoracchius from its treatment in the books seems to be the Sword that requires the rarest kind of individual to wield it. This also seems to be supported by Arthur Langtry's reaction to Michael in Proven Guilty.
Just his internment made him suffer the same as his victims over the years. It's one thing to have regret for his actions and empathy, but he gets the full treatment of exactly what they went through, which is something a bit more special as far as suffering goes. Eventually, Butter's version of Fidelacchius will be wielded to kill his demon, making him a vanilla mortal.
But where does the British Prisoner fit into this?
Well, we've got a negative from the author that it's Merlin. The only other mythological figure I know of that's supposed to be sleeping under a magical island is Arthur. That whole 'Piss Off' comment had the vibe of someone trying out a slang term they weren't terribly familiar with...half remembered language lessons from Merlin perhaps?
If Thomas is being set up as a character to take up the Sword, then having an extended chat with the most legendary wielder would be an interesting thing to consider, and it would immediately set him apart even from Michael's legacy. But I would expect this process to take a book or two to end up resolving, and Thomas to be on carbonite for an extended period of time before someone makes the connection with Fidelacchius and ends up freeing him.