Did Billy really play Myra in Hans's vision?

Hans: My wife is sitting on a chair someplace. Some gray place. I thought she'd be in Heaven, but she's sitting on a chair with a bullet in her head. I thought they'd have cleaned that kind of stuff up.

Marty: Maybe you've just eaten too many hallucinogenic cactuses tonight, Hans.

Hans: Nothing to do with the hallucinogens.

Marty: But you've just seen Myra on a chair with a bullet through her head.

Hans: In some gray place.

Marty: England?

Hans: It seemed a lot worse than that.

Marty: Wow.

Then, later on:

Billy: Whoa. Whoa. Time out. What's all this about doubting a lifelong belief in the afterlife because of a psychedelic cactus you just ate? Hans, what the heck?

Hans: I met Myra. On the ridge. She had some things to say.

Billy: About the afterlife being non-existent or something?

Hans: That was the gist.

Billy: No, no, it might have sounded like Myra. But you know why? Now don't get mad, but you know I can do Myra's voice pretty good. Yeah, I snuck up there a little while ago and I pretended to be her. I started saying all kinds of crazy stuff.

Hans: Hmm? But what specifically did you say? About the place you were in? The place Myra was in. Huh? How did you describe it, specifically?

Billy: You mean specifically?

Hans: Yeah.

Billy: I just kind of said it was all kind of... I just kind of said it was all kind of gray and shit.

Hans: No.

Going by this, and my initial impression upon watching the film, I got the feeling that Billy was telling Hans what he thought he wanted to hear in order to make him stay. The whole idea of him impersonating his dead wife is B.S. (there's no reason to think that Billy didn't hear his earlier description of the vision) and, as you have already suggested, this is Billy's desperate attempt to make Hans stick around for the inevitable showdown.

The fact that Hans says 'no' and leaves suggests that he has had a cactus-fueled epiphany, and he chooses to wander off to face his fate alone. He is lost without Myra and ready to join her, wherever she is, as is evident by his flippancy when faced with being shot by Paulo.

Billy is lying. There is a chance that he did say somethings to Hans while doing a Myra impression but Hans described a vision not just a conversation. So I don't think that Billy ever said anything to him. Billy has one good trait. He is a good friend. When seeing Hans in turmoil and doubting his lifelong faith he lies to Hans. Another clue that he is definitely lying is that he says "You mean specifically?" after Hans has asked him to say what he as Myra specifically said to him. Billy probably just says what he actually thinks the afterlife looks like, "Just gray and shit". I don't think Billy says this to keep Hans with them for the shootout but rather to help Hans keep his faith. Much like he knows that script writing helps Marty he knows Hans faith helps him and he actually wants his friends to be happy.

Why did Hans go off into the desert on his own though? It's because he was in a no win situation of his own. Either he doesn't believe Billy has a loss of faith and is insulted that his friend lied to him, or he does believe him and is mad that his friend played a joke on him using his murdered wife for his choice of prank. Either way he no longer wants to be around Billy. Billy probably knows this before he lies too but sets himself up to be one to blame on purpose so that Hans crisis of faith is adverted. He'd rather have his friend hate him but continue to keep his faith, even though Billy doesn't believe it, because to billy Hans faith makes him happy.

Yeah, the way I see it, Hans knows there was no way he imitated Myra's voice but it just confirmed that indeed another persons view of the afterlife was that of his own. That it is all just grey. He lost faith at that moment, leaving them both on the mountain. He refound his faith whilst dying. 'It isn't grey at all.'

Also note Hans had recorded the message left to Marty while he was alone on the mountain. His ending of the story was that of the monk. A notion of self sacrifice for the greater good. He knew that pulling that very tape recorder out of his pocket would get him shot. He sacrificed himself after having nothing else left in order to alarm the police. It was never guaranteed that this would save anyone or 'end the war'. But as the monk had stated; 'It might.'

This film has so many sub texts and layers, as previously stated by Hans in the scene where Billy is explaining how he thinks the film should go. It's really very clever, not to mention the meta level of the screen play concept as a whole.