The legend that is Dev was indeed on fine form tonight, but if proof was needed that he is a bit of an idiot, then ........... Dev and his father Aadi ?
I do not think Stephen altered the original letter. I think he is trying to do something good for Sarah, or at least something he feels is good. He had not counted on Sarah wanting to confess out of guilt. Just because Stephen is a slimy, sneaky, lying, cheating, swindling murderer, does not mean there is no good in him, Sarah is his niece and he wants to help her, even if in the future he might use this to his advantage.