1. Agree that French intervention is unlikely A nice token offer is really all they need to give here. In the end, the French get nothing
2. Reinsurance treaty comes much later. The Russians aren't making any promises here
3. This is not Bismarck's call. This is the King's call. Moltke thought he could win a short war but not a long one. The Prussians do win but they aren't assured of that beforehand. Whichever side Russia joins wins unless someone else interferes. The King can make a deal with his nephew and take down the Austrians or his nephew might take the no and make an offer to the Austrians.
4. True, Belgium neutrality is part of the country's creation. The treaty gives the parties the right but not the duty to protect it. Making the offer is typical, crass politics. Napoleon might take it or not, the British aren't likely to do anything without a continental power
5. Which brings us back to Austria. Obviously, they aren't going to announce such war aims and will spring them only after they have won a complete victory. given the disparity of forces and how things played out OTL, that is likely to happen very quickly
6. The British might not like it but how do they intervene? The Crimean War had acheived little against Russia and the French aren't going to be helping. If France objects, its objecting to the growth of Prussia far more. Prussia is also easy to get to and the French would, if they intervene, intervene against the Prussians not the Russians. This isn't in British interests. The last thing the British need is the French pressing deep into Germany In 1914, the British care far more about the balance of power of Western rather than Eastern Europe
Really, to fight Russia, the British would need the Prussians. There is the possibility but any expansion of Prussian power is going to start sending off alarm bells in Paris
Remember that after Sadowa the king wanted to push on and reach the gates of Vienna, and the generals were not much behind in clamoring for a continuation of hostilities. Bismarck managed to convince everyone that it had to be a short war, and the real danger in continuing was to bring in the other Powers.
If there had been a coalition including Prussia, Russia and Italy, declaring war on Austria the outcome of the war would have been known to the man in the moon
It was not in the interest of Prussia to upset the applecart of continental balance, where Austria was the linchpin even if they were not anymore the Austria of 1815. As a matter of fact, the issue of a Russian intervention in the war was never discussed: the Russian themselves were less than keen, and the geopolitics of the czar were turned more toward Central Asia than Europe. Bismarck got what he wanted, a commitment that Russia would not intervene. The king would not have intruded: why should he? after all Bismarck had already pulled a few rabbits of his hat, including the successful war with Denmark and the casus belli for the war with Austria.
I don't believe that Bismarck had already planned the future: not as early as 1866, and anyway even the way the German empire was born proves that it was not a planned outcome of the Franco-Prussian war. However I do believe that he had already in mind a balance of power based on the alliance of the three conservative states, Russia, Austria and Prussia, who with alternate fortunes had been the guarantors of stability in Europe for more than thirty years. In the end he failed, since even good old Otto could not mediate between the contrasting goals of Austria and Russia: he certainly did try hard, though.