I would like to go along with your definition, but doing so muddies up the waters too much.
Say you took dirty pictures of me. And then you told me that you would send them to my dad if I didn't have sex with you. If I complied, would you be a rapist? No, you would not. Because rape requires violence. It the act of taking away someone's sexual freedom through violence or threat of violence. If you raped me, I would feel traumatized not just by your lack of my right to choose but likely more so by your violent attack. If you blackmailed me, I would still be hurt, but less so, because the act itself would likely not be a violent act, because I would likely be somewhat willing participant.
Also, don't forget that Moss is not threatening to do anything to Bernadette himself, just refusing to help Maytag without sex. He never implied that he would hurt Bernadette if May didn't sleep with him. This is an important distinction because unless he actively helped kill Bernadette, he would not be directly responsible for what the Thin Man/Melter did to her.
I would like to add that I respect your opinoin and that you bring up valid points, just not ones that I personally hold to. No matter what we choose to call what Moss is doing, I think both agree it is very very wrong and a likely sign that he has jumped off the deep end from morally ambiguous and distasteful action to unforgivable depravity.
