Mayhem 18 can be done, with genuine same lemming, but not 100%.
Apart from one steel glitch (the minor, dig-at-the-edge kind, not the major break-the-whole-way-through kind), there's nothing special in the solution. Just clever use of skills.
Also: To save anyone else trying, Mayhem 12 and 14 are both impossible solely due to the time limits. Take that out of the picture, and they can be done.
Mayhem 16 and 27 can be done. The replay attached works on both, even though the filename says 16.