Here is my feedback to your questions in no particular order.
1. I think there is no market for a 9.3x72 or 9.3x74 double rifle, especially not at $25000. These rounds are so light they can be built on shotgun boxlocks and for $6000 you can obtain just such a gun. You'd also be competing with fine vintage drillings at fraction of the cost that fulfill this market.
2. I believe that high dollar guns and handloading do not always go hand in hand. I suggest you stick with calibers that are able to be procured off the shelf and that your guns are regulated to off the shelf product. I can't understand/envision a market for a double gun that shoots anything less than .375 in the pricing you're intending to hit.
3. Instead of attempting to cannibalize a market that you'll be unlikely to penetrate heavily (you're competing against Krieghoff and others), why not create a new market? Think about the idea of a modern homage to the classic British Paradox gun. I would think that someone could even make screw in chokes that feature the ratchet rifling so you could truly go from an upland gun to a large game gun easily, with better accuracy, all the while carrying a very traditional British arm that would accurately convey the AG&L mystique. I do not believe any firm makes a paradox gun today and it is ironic, since a paradox gun today could be very powerful and versatile in a way the originals could not. (e.g. screw in rifled chokes, heavier proof loads, etc)