Agreed that keeping Napoleon III in power benefits the CSA. And if keeping Austria stronger relative to Prussia (for longer) is also necessary, then it would be necessary to either prevent the Austro-Prussian War or make such a conflict a victory for the Hapsburgs. And it strikes me that the best way to do either of that is for Nappy not to give Bismarck any assurances of neutrality. Which may well be an effect of the CSA winning the ACW.
I agree that the mere appearance of a French-Austrian entente in late 1865 or ealy 1866, like Napoleon assuring his help for Austria if Vienna cedes Venetia to Italy, would probably lead to Bismarck rethinking his plans; probably settling the acute conflict with Austria (over the fate of Schleswig-Hostein) peacefully.
Hm, there was a proposal by Napoleon III. that Austria could be compensated for the loss of Lombardo-Venetia by the acquisition of Moldavia and Walachia, thereby securing the mouths of the Danube.
Aussume that this reaches the stage of concrete discussions (like a French-backed Austrian proposal to buy M and W form the Porte, perhaps even with money from the Italian proposal to buy Venetia.) In this case Russia will have completely other worries than whatever happen in the US.