I think even if 16 ends up having unintended solutions, they'd be so complicated that they might as well be official!
I definitely prefer levels that are concept or insight driven, which force you into taking a step back to look at what the level is asking you to do and what interactions of mechanics that might involve. The last three are very much in that category, and force you away from the sort of fingertip aha moments that you get when you're just pottering around the level seeing what's what. Even so, it's always worth trying to avoid the harrowing effect (at least in linear puzzlescript games) where each level feels impossible, takes everything you have, and then the next one seems even harder :)