I'm a little late to this, but it feels like the level needs to make its intended usage of shimmier more essential to the solution so that it cannot be so easily bypassed. After all, we are not talking about a mere jump or turn around like a jumper or walker, we are talking about the ability to go along the ceiling to get past things like gaps and traps you would otherwise have to build/bash/disarm/etc. to get past. How is that if a player used the shimmier elsewhere for a skill cancel, the part that actually requires the essential use of shimmier can then be so easily bypassed?
In other words, seems like generally speaking, the shimmier should be less backroute-proned compared to a skill like walker. If the player "wasted" a shimmier for skill-canceling or delay purposes, the consequence of then not being able to actually shimmy across a ceiling somewhere as intended by the solution, should be severe enough to prevent the player from solving the level. Whereas it seems a bit more difficult to have it like that for a walker.
It is completely unavoidable that if you allow lemmings to attempt to shimmy at basically any time, there will be many more opportunities for player to use the skill for skill-canceling or delay purposes compared to the showcase use, simply because the showcase use requires specific terrain setup in the level to make it useful (or in some cases, even possible at all) to shimmy across a ceiling. Whereas skill canceling or delaying can pretty much be done anywhere. So one could consider limiting ability to assign shimmier for the skill-canceling cases (or more accurately, for cases where ceiling is not reachable), although at this point, you will also likely end up breaking many other existing levels that do rely on this behavior.