The problem with the backroute is, that one can start the miner much later and it still works (see attached replay). So the point where the miner finally breaks through the earth wall can be pretty much the same as for the intended solution. Hence we would have to add hazards within a very thin layer at the top of the earth platform (at least at some places). I don't like this either.
...to enforce that the crowd may only turn once at the left edge of the earth platform. But I didn't find a good way to do so. Placing a small sawblade there and giving 12 jumpers or walkers to turn the crowd introduced backroutes. Other ways required bashers/miners/diggers/builders/platformers which usually introduce backroutes as well, unless one is extremely careful (which I wasn't in the versions I tried myself).
A time-limit gadget (for lack of a better term) like what you proposed could work, but especially in a community-levelset context, you may want to check with Simon over why such a gadget would really be so much more acceptable compared to an actual time limit, the thing that was removed.
Yes, I feel the same and would prefer to do without the time-limit gadget.
Alternatively there's also the option (though probably not one many would be happy with) to demote the level to an earlier ranking and allow some of the backroutes to exist.
Ignoring my first tries at the level when I was still very inexperienced (and using CustLemm), then finding the backroute took me approx. 30 minutes. Finding the intended solution on the other hand took me another few months. So I would very much prefer to keep the backroute impossible.
Finally solved the original version of Trading and Cooperation, which turned out to be very similar to my solution with the wrong hatch order. I am not totally happy with the current version, because execution is pretty tough, especially two points:
1) Placement of the blocker: One brick less and the builder does not close the gap to the upper platform and one brick more and the jumper may not turn. Why not make the middle gap slightly larger and let the platformer become a blocker after placing the last brick? This would be much more natural.
2) After having placed the blocker correctly, the jumper placement is extremely tough to get right: She has to start at the very first moment it becomes possible without hitting the ceiling, otherwise the jumper will not make it to the blocker or jumps over the blocker. After turning around she will barely land at the outmots pixel of the lower platform. With all the required (almost) frame-precise assignments, it is far from obvious that this jumper trick works at all! In fact, this was the reason why I couldn't solve the level for a very long time: I thought this trick was impossible and looked for alternative approaches.
I am not sure what the best method is to simplify execution here. But I found that letting a lix jump, who moves in the same direction as the platformer (either while still platforming or jumping over the blocker), is a lot easier to execute.