Okay, why a British accent? Because it's an easy lie. EOS.
This is what you're quoting and I think you are misreading it. It's gibberish if taken straightforwardly. You can't change history by shaping the future.
Here's how you might do it for a story. Who fixed LC? If Harry did it, how do you escape the paradox? Because someone in the future told you you needed to. And it would be helpful if you changed the future by changing that person now. So if you changed Lasciel's shadow, you produce Bonea who can put the pieces together at the right time and send you back, to do everything you might need to do. It's established that Lasciel knew everything, all of Harry's secrets. You shaped the future by changing her and she changes your past. This closes the loop. Bonea knew because Lash broke Little Chicago. And she could sabotage LC because she could make Harry see exactly what she wanted him to see. Like Sheila.
That's my solution to the bootstrap paradox. Dazzle them with footwork.
Is it? Seems like it would have been easier not to talk to Dresden, not to mention Dresden's not exactly a great actor at the best of times. If he had dramatically improved his lying, why not talk in another language? He also clearly had enough psychic power to throw Dresden's mind out, even from his cell. He didn't have to talk to Dresden at all. He didn't even WANT to talk to Dresden.
To be fair, all time travel explanations are mostly gibberish. It's only possible (as far as we know) at a quantum level. And not in any way that we could use to effect that sort of change. I am not stating you can change history by shaping the future, and I don't think Vadderung was either. That would literally be going to the future to change the past.
What I believe that quote is saying, and I could be wrong, is that if you wish to change what might be (the future) it's a lot simpler to change things in the present. This requires a few elements. You would have to know what the likely future is and be able to make reasonable predictions about it. You would also need to have the tools to be able to change the pivotal events that would lead to those future outcomes. Like diverting a great river with well placed rocks or newly dug trenches. Somewhat like what Mab said in Battle Ground to Harry...and she said there was always a cost. All of which assumes the future can be changed in the first first place...isn't destiny supposed to have a say? Yes, I believe Mab said destiny is a stone-cold bitch.
You're LC proposal is an interesting one. But still a time loop, unless I am much mistaken. Which is a sort-of paradox I think. But perhaps I have misunderstood it. Harry changes Lash, which causes Lash and Harry to have Bonea, which causes Bonea to eventually become the Archive who eventually tells Harry to go back and fix LC because he would die then otherwise...and he then comes back to his present. Yes? Plus whatever other fixes in the timeline were needed.
Here is my theory on this, and how Jim was able to fib on it while telling the truth. I do believe that person is either King Arthur, or the original Merlin. It's true their English would be way different. However I believe the Island allows Harry to understand anything that Harry wants to communicate with. There were all manner of things trying to whisper to him. Chances are they have been there for so long, and don't even know English, and yet Harry understood them.
If the person was say German, or Chinese, I think Harry would understand them but hear their accent even if they were speaking there native language. It's all tied to the Island, and that no words are actually being spoken at all. It's a form of telepathy. So they think it, the Island deciphers it, and Harry understands it.
Well I think you're right about the psychic translator. But I will add something. Almost every British character, or character/being with a British accent, is a villain in the series. Likely this is a leftover from a well-known trope. In point of fact, even some of the British characters that are not out-and-out villains...are still suspect.
So even if this person is Harry, or King Arthur, or whoever...they have a better than not chance of being a villain.