The chase scene could go on just fine without Lurtz (since he wasn't in the book anyway ;~D), but I don't think they will complicate matters by bringing Mordor Orcs into it. Not sure it matters to the story if there were Mordor Orcs... Sauron finds out about Saruman's treachery from Pippin in the Pilantir, not the Orcs, if that part of the story is even gonna be included. Not sure how much it matters... You think it does?
Yes.