I'm happy to release the second beta version of the game! It's available in my first post.
I've really appreciated all of the feedback I've received; it's helped make the game much better.
Most importantly, I've changed the kneeling shooter levels so they're more satisfying. I've ensured about four pfpixels of empty space on each side of them. I've found through testing that's juuuust enough room to get off a shot and get out of the way if you're precise and/or lucky. So no more getting dealt an immediately unwinnable canyon! I decreased the speed of the shot a bit, too, to make things more fair (your shots and the enemies' are now the same speed). I also increased the size of missile1 and darkened the shooter sprites a bit, so I think their bullets are a lot more visible.
Another concern was that some of the shooter levels could be completed simply by going left. The game now places the water a minimum of two rows above or below the player. This makes it much, much more likely that you'll have to contend with at least one shooter along the way. I think this change has made the other levels more interesting, too.
The canyons now draw much faster. This is purely aesthetic; I didn't save any code space. But it makes the player have to wait less in case a canyon needs to be redrawn. One consequence of moving the water two or more rows away from the player is that sometimes there simply aren't two rows either above or below. The game checks for this, and creates a new canyon if that's the case. Originally, the game redrew a canyon if it couldn't find a clear path to the water; in worst-case scenarios there are two separate canyons, one above the other. By forcing certain positions for the water, the game now redraws canyons more frequently. The instruction text that I wrote tries to explain that in terms of the story: "Every so often, you’ll enter a dead-end canyon that has no clear path between you and the water. If that happens, you’ll automatically back up and find a new canyon with a better trail." Admittedly, it's not a strong explanation, but I think for the canyons to provide the most fun / challenge, a couple per game will have to be redrawn. I wish I could avoid it, but unfortunately I think other solutions (such as putting the player at the beginning of the first path and the water at the end of the first path, thereby always guaranteeing the player can reach the water) make the game less fun.
I feel like I'm getting pretty close to completion. But I'd love to hear more feedback. I'm still interested in the balance of the rules--speed, needles, score, lives, etc. I have <10 bytes free in each of my banks now, so adding new features is unlikely (though there's so much more I'd like to do!), but numbers that already exist can easily be changed. I'd be especially interested in hearing from anyone who tries the Expert difficulty--I worry it makes the game unwinnable (I haven't beaten it yet), but I'm not sure what I should change.
Again, I'm really grateful for the comments so far. The game feels much more complete to me now.