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.