I dunno, I kinda like how it cut off and the audience simply recognizes Saruman's inevitable doom.
Yeah, in the legendarium Saruman attempted to make use of the Orthanc-stone around T.A. 3000 and lost to Sauron in a battle of wills. I don't think we needed to see a physical confrontation between them on the borders of Mordor.
I’m not sure. Really, he never goes to Mordor, his downfall is using the Palantir so much that Sauron was able get into his head