I think the problem is I’m just bad at kaizo shit. I took a break and tried it again, and this time I had more luck. I realized you only phase through the turtle when it has just rebounded off the music blocks a split second sooner, so you have to get a better bounce of its shell. I still think the coin indicator should be over the left music block and not the right, though.
On another note, I still haven’t got the hang of shell jumps, so I keep grabbing the beetle shell right before the check point, hurling it against the on/off switch, and hitting myself in the face with it.