You have a point there, star wars is set in stone. Marvel is just a universe to play with.
Only Episodes I - VI are set in stone. Everything else is.... maleable, for lack of a better term. The different levels of Canon are in place for a reason. G-Canon is the only Absolute. T and C are there to show that it was approved by Lucas, fits with the general plot of the movies, and doesn't stray too far from the spirit of Star Wars. S, and N are there to deal with discrepancies. For example, the account of the Clone Wars given in The Thrawn Trilogy was wrong, and was relegated to S-Canon as a false rumor of what really happened. Since it could be viewed that the Clones went "insane" when Order 66 was given, it had just enough credibility to stick around as an S-Canon rumor. Leland Chee has the unenviable task of making all the pieces fit as much as possible. It's hard when the puzzle keeps changing shape.