Tip: Switch palace switches normally disappear if you replay the level. However, if you insert them as direct Map16 and use the hex-edit at $00EEB2 in the ROM map, you can replay the level without a problem.
I've had tonnes of fun with this custom sprite so far.
The only issue I've ran into that's preventing me from progressing in building a setup is that if a blue koopa catches one of these shells in a kicked state, the hit count will decrement once the shell is stopped. This makes it impossible to steal the stopped shell from a blue koopa.
At the risk of being vain I like to think I was responsible for this one because I asked you if yours was a custom sprite and you had said it was actually using blocks that keep resetting the koopa's status to kicked.
Basically I'm super glad this exists, thank you!!!