1. Proof the national treasure isn't the Rio Poneglyph?
2-3. Perhaps he needs the Ope Ope no Mi to have his personality transferred into a Celestial Dragon to gain access to the Poneglyph. Perhaps he needs to have the Ope Ope no Mi because the Poneglyph is locked away in some impenetrable safe, vault or barrier, and he needs Shambles to get it. Perhaps he needs the immortality the fruit gives to gain access to the Poneglyph.
The information Doflamingo has is what the National Treasure is. It's possible that if the National Treasure is the Rio Poneglyph, or another Poneglyph, that means the information Doflamingo has is that the Rio Poneglyph exists, and where it is, but not what it says. It's possible he knows that the Poneglyph tells the location of one of the Ancient Weapons, how to use it, and how to get to that location, because it has the Void History on it which reveals all of this information. Perhaps you either need to be a D. or an Ohara scholar to decipher it. This could also be why Doflamingo wanted Violet's powers so bad. Since Doflamingo isn't a D., and there are no Ohara scholars left alive aside from Robin, he needs her clairvoyant powers so she can read it to him, and he can find the weapon.
The thing is, we don't have enough information on what the National Treasure is to say this isn't right. Saying the National Treasure is a Poneglyph is just as correct as saying it's not, and also just as wrong. None of us can make that call yet, and rule out that it is or isn't a Poneglyph or related to the Will of D. and the Void Century.