Once Upon a Time in the West (1968) - 2 questions

Question: There's a few things I didn't understand in this film: 1) What's the deal with Jill? Did she really love Mr. McBain or did she just marry for money etc? 2) After she sees the McBain's bodies, why does Jill search the house? Is she checking to see whether anything was stolen? 3) When Jill meets Harmonica in the barn, why does he rip her dress? 4) What's whole thing with Jill and Frank near the end? What exactly happens?

Answer: The basic answer to all of these questions is that she is a "freelance" business person herself - a very successful prostitute from New Orleans who headed West to start a new life. Many of the interactions you mention are Jill either being confronted by past customers, or her trying to gain information and/or the upper hand amongst her new neighbors. It's actually pretty hilarious that "Manuel" (Jason Robards) has to keep telling her at the end to be willing to take a pat on the bottom to appease the railroad workers. It's a good bet that 30 seconds after he died, she had a price sheet on the door of the station, and was reinforcing the springs of her bed. Doh.

Question: What's the significance of the discordant chord Harmonica plays after Cheyenne tells him to "watch those false notes"? Why does Cheyenne react like he'd heard something very unnerving?

Answer: A better question is: "why is he doing it in the first place?" Harmonica knows exactly who he is after. Harmonica does it for Cheyenne, though, just to assert himself - as they say, to "pull his chain", to annoy him.