I feel like we're still missing something here. I like platforming a lot and was really excited for this type of mission before it came in, but it's so punishing that I feel like I'm pretty much excluded from doing it, at least without greatly reducing the difficulty level, and I'm not sure if it would really be fun then. Anyway, the secret mission solution ---- design the mission type for really hardcore players and then hide it ---- seems like a shame for a cool challenge that I think at least has a close cousin that would be accessible to all players.
Thinking about how these types of levels work in games where I've enjoyed them ---- VVVVVV I have a really strong memory of ---- I think the thing is something X said a while back about AIWar; it's really a puzzle game. But that means if you're not super-skilled you have to throw a ton of lives at the level to learn the route and some tricks to make you faster, which is a really unpleasant (although, if I understand rightly, not actually harmful to your game) thing to do in AVWW.
I'd suggest, then, keeping lava escape as it is now as a secret mission, and adding either a new primary mission type or a secret mission type which appears in less inaccessible places where being caught by the rising wall sends you back to the bottom without killing you, so less skilled players like me can make a lot of runs and learn the route without getting as frustrated. I would support making the environment in these missions more deadly --- with a lot of open space and lava pits --- since you'd have a few goes to figure out a way past these obstacles.
I don't think it would be terribly difficult to square that kind of thing with the game's generally realistic theme. Off the top of my head, it would make sense to me as a rising column of wind that sucks in anything it touches, or as a Stalker-ish "anomaly" that just warps you to the bottom.
