Oh, keeping him quiet was only for while he had Sidious on the comm. I'm sure he'd let Piglet talk in private. The stammering, on the other hand....

I think I concluded Magneto was there looking for the New Mutants, who had hared off into the wrong universe again, and that they'd all end up on Coruscant. Which, er, I might have already said in another comment. However, the plot-planning got that far and then fizzled.