This is really the only explanation, imo, why they are continuing. They started the investigation, found some things that needed explanation but the club didn't provide the proof to support the explanations because they aren't required by the PL rules, or by UK law so to do, it being information held externally. So what do the PL do? Finish the investigation without a proper conclusion, which is a terrible situation for a governing body? Or continue with the disciplinary process, incur huge legal fees, and then "lose" the case, which is a terrible situation for a governing body.
The club has stitched the PL up good and proper over this, I think. We really have some clever people running the club.