I’m afraid that you are more than a little bit confused regarding structure of the Russian exports at that time. Grain was not atop item on the list (items needed for the ship building were). Odessa became important much later and the same goes for the exports through the Black Sea (access to which was during that period quite often blocked by the Ottomans. Russian main ports were St. Petersburg, Archangelsk and Riga.Well, I don't deny that it was quite weak, but quite weak support is surely better than no support, which Napoleon met in central Russia, isn't it? And while talking about CS and restoring it, you forget one important thing. Grain produced in Ukraine was one of the main Russian export products to Britain at that time (Odessa was later nicknamed by historians: "European Hongkong", because there was that much grain trade with British being done by it), so cutting this export by setting up a puppet Poland in this place doesn't sound like a bad idea to accomplish the purpose of strategically weakening Britain and cutting it's trade gains.
Conquest of Ukraine would, as I already said, benefit the Polish nobility but in a process of doing them such a favor Napoleon would have his flank and rear exposed and the whole affair turning into a prolonged war with no clear outcome and a need of a massive long-term commitment, something he already had going on in Spain and tried to avoid in Russia.
Last but not least, an idea of cutting British trade gains is great but would not work: Britain had a negative trade balance with Russia. 😜