Once Saruman is in play and at least one companion is in Fangorn, a muster die can bring Treebeard into play. When this happens, the rules say to place an ent in Fangorn. Does this mean to place the silver Treebeard figure as well as a blue Ent in the Fangorn region as soon as Treebeard is mustered? Or is Treebeard the only Ent figure placed, and the other Ents can only be placed after the Voice of Saruman?

Just place a blue one. The silver one is used in the battle for Helm's Deep, iirc.

That's correct. The silver Treebeard figure is used only in the Battle for Rohan campaign game. Only the regular Ent figures are used in Twilight of the Third Age expansion game.