The first thing you must do if you want to discuss a possible conspiracy is list all the conspirators and their motivation for maintaining the conspiracy for all these years. When I see such a list then we will be able to begin to discuss it.
That's actually putting the cart before the horse, from an investigative perspective. First *the evidence* must be proven to reveal - in a Peter Gill/John Douglas world, not the Alice in Wonderland one of the Perugians and the guilters - a scenario that cannot be innocently explained. Given that any actual scientific results indicative of what Diocletus believes with regard to the downstairs apartment have surely been destroyed, this is a tall order bordering on impossibility.
But if it *could be proven*, the evidence would drive the theory, and then the individuals with responsibility over the proper care and release of the evidence would be identified, and fall like so many tin soldiers.
My own opinion is that, rather than any conspiracy per se, the facts of this case point more to political and sociological phenomena underpinning any sort of "cover-up." There's ample evidence that such behavior is business as usual, and that many Italians would not recognize what we perceive as gobsmacking amorality as anything other than normative.