Thank you for the new chapter Dom, I loved it.
So, at the moment I personally believe that what Brian told Frank was a lie, a made up story to hide the real secret, which would cause a lot of trouble for the Martin parents, in case it came out. If Oliver and David are one and the same, I have a feeling it's David that is the "original" one. There must be something else than just David being an imaginary friend, right now I'm not believing it for a second.
Possibly Frank's dad will have a big role in solving all this, looking forward to it.