Doubtful. Their writing styles were different. And Grant was noted during the war as a clear and direct writer - many of his officers noted that when he issued a set of orders for a battle, everyone understood what the plan was and what their role was in it.
I, too, think it’s doubtful. If for no other reason than the timing. When Grant decided to write his memoires, he’d just been diagnosed with his fatal cancer and had only months to live.
Not only was this terrible for him for the obvious reasons, but he was broke again. Which meant that he had to do something, fast to generate some income to care for his family after he died.
Catton’s biography of Grant makes it clear that Catton was most impressed by how Grant managed this feat. IIRC Grant had only months to live, and was in constant pain for much of that time. And he still managed to get several hours a day done writing the manuscript, until he finished it shortly before he succumbed to the disease. I just don’t see any publisher taking the time to have much editing done on the works, with that kind of timing.
ETA: Yes, Twain was involved - getting the ball rolling, and setting up the contract; I just don’t think he had the time to actually do anything to affect the content of the memoires.