I see "Pluton" written there...
I always thought the reason that they were after him was because it was an artificial mystic zohan fruit while the other artificial fruits are just normal zohans. If they could mass produce the mystic zohan they could make hundreds of times more than before. Mystic zohans are rarer than logias and a tournament for a logia fruit that incentivised some of the stongest pirates in the world.
Sry if it sounded harsh but this has just been stated waaaaayy too often...
I agree, the interest being in mononosuke's fruit is unlikely. Mononosuke's importance is evident ever since kinemon revealed a bit of their history. Kinemon was traveling with fellow samurai to zou and mononosuke was with them. In the way they got attacked and mononosuke got accidentally taken to ceasar's lab. Now, what exactly were those 3 doing? Mononosuke is a brat, if those 3 new there was danger then why take him? I doubt they were attacked for no reason either, doflamingo is not that impractical as far as we know. The only reason for mononsouke to have been there is for him to be the mission to begin with. It was their mission to take mononosuke to zou for whatever reason. Doflamingo got in the way for whatever reason. I would argue this pertains to kaido though. Kinemon and mononosuke reacted quite heavily to kaido being mentioned as they were traveling with the strawhats to dressrosa. So odds are that it is kaido that is interest in mononosuke and with doflamingo being a close partner to him they probably left it to him to capture him if at all possible.
You still haven't explained why the blueprints say PLUTON on the front if they are not, in fact, the blueprints for Pluton. Until you can do that I'm just going to continue to assume they are Pluton's blueprints and that the shipwrights' intentions for keeping them was so they could build another one to counter the first rather than them being completely different blueprints that would nevertheless result in a ship as powerful as Pluton even though said ship would obviously never have been built before so how would they know?
While Franky never says directly there that those are the blueprints for Pluton, he responds positively when Spandam asks if they are on the page I posted. "It's the real deal.", he says. Even if the purpose of those blueprints is to build another weapon, that weapon can be built in the same way as Pluton itself, based on the blueprints for Pluton.
Really, what's left of your argument is "nobody who knew about it said directly that those are Pluton's blueprints", which is ridiculous. If you really think so even after all that's been said, then feel free to hold on to it; I'll just laugh it off.