Your statement would mean that you did indeed test some levels in Superlemmini, including 10, to make sure they're solvable, just not thoroughly, so now you need to make sure you do it for all your levels. You did good for your first attempt at a level pack, but moving forward take the lesson of level testing for solvability to heart. Remember, just because the levels work for some engines doesn't automatically mean they'll work for all, which is why it will be necessary to tweak some levels so that they still remain solvable in a particular engine.
Well that's exactly what steel is: terrain that you can't destroy with destructive skills under "normal circumstances." Most of the time, you can destroy part of it only if you take away terrain that's destructible at the same time, i.e, you bash/mine/dig steel and normal terrain at the same time. Neolemmix removed all known glitches, including steel and hence why steel is completely indestructible in that engine. Solutions that involve destruction of steel generally are backroutes that allows one to save more than the necessary amount, but most of the time such solutions are used in challenges, e.g, the lose 2 solution to Mayhem 29 Save Me.
