Ah ok, didn't realize he sent you his. He had basically the intended solution or at least a tolerated version thereof. I actually had tried to play my copy of his C++ replay in D-lix but it didn't seem to work for some reason, though it's possible it could be something stupid like the replay having been edited to point to a different file path (eg. to my own copy of the level file that I may edit to try things out, say), I'll check again later.
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For "Oh No, Not Again", for the backroute made possible by not having a time limit, can it potentially be eliminated more directly by changes that would force the miner to not be able to start mining around that area (eg. he'd end up having to navigate around a hazard after he's done, say), even if at the expense of making it more obvious what probably doesn't work? 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.
There's always the option to scrap the level, after all, that is bound to happen for a few levels once the decision was made to remove time limit support in D-Lix, and I seem to remember we aren't exactly starved for replacements. 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.