If the One Armed Man knew that Leyland was the killer (the car scene) then surely in the series he would just let Cooper know?
I guess he did let Cooper know in his dreams. Or rather his arm, The Arm aka The Man From Another Place (the dwarf), gave him the clues he needed in the show. You'd have to parse the differences between Gerard, MIKE, and his arm, and understand how each are motivated. Really, there isn't enough in the film or show to give us a full understanding of these characters, how they interact, and how or if they're different from one another (i.e. are the dwarf and MIKE one and the same? Does Gerard understand or remember everything he sees when he's inhabited by MIKE? How effective are Gerard's drugs in warding off MIKE's spirit?)
shareI think they're dream signifiers. Obviously that doesn't mean that they don't also exist in the ordinary world; after all, we do see Gerard and he clearly has had some kind of accessible contact with the twilight world. Still, within the dream I think they might sometimes represent other people, or even concepts, so that knowing more about their waking selves might or might not help anything. Maybe more revealing to know more about the dreamer and how he might interpret them.
Of course, one thing that is known about Philip Gerard in his waking life is that he travels around the area as a salesman. Assuming a fictional landscape that is as sparsely populated as the real Pend Oreille County, WA that Cooper describes on his drive in, Gerard must have to cover a lot of territory to make any money.
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Gerard: [...] but you know something he ... he kinda looks like somebody.
Doesn't he?
--Episode 4
Truman: You haven't seen Gerard the shoe salesman, have you?
Hawk: The shoe salesman? [shrugs no]
Cooper: The one-armed man?
--Episode 10