There's one thing you missed when you were summarizing the act in America, which is that it applies to all the people who did that to Sergei, plus all other gross human rights abusers.
I don't believe that in the current regime the Russian government will ever create a proper investigation to bring to justice the people who killed Sergei Magnitsky. However, if you create a consequence for this and other crimes, then people in the future may decide not to commit those crimes because there's a consequence. Right now, if you're an immoral person, where you get a full benefit for committing a crime and there's no cost to committing the crime, then you'll commit a crime. If all of a sudden there's a chance that your assets will be frozen and you can't travel, you might think twice about committing a crime.
My hope, as a campaigner for justice, but also for Sergei's legacy, is that the law with his name on it will save lives in the future, because people will be afraid to do these types of things.