Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
Using cluster sprites for the cave in generator is a great idea. I mean, the reasons are obvious: The amount of the blocks falling down is relatively high.
That, however, causes a couple limitations:
They use a simplified interaction.
The biggest one is that the block interaction is only focused at one point which means that the rocks can actually fall through blocks even when they shouldn't with a "proper" object<->sprite hitbox
Another limitation is that only tiles with an acts like setting of 1 (which contains tiles where you, sometimes only sprites, including 1F0) or higher can block the rocks. The problem is that coins and invisible ?-block won't block them when the blue P-switch is activated and in contrast, Rocks see brown blocks and munchers as solid tiles even when the blue and silver P-switch is activated, respectively.
Layer 2 interaction is also non-existant for them.
And finally (the biggest limitation), be careful that cluster sprites aren't really made to be 32x32 large. This means, instead to use a hardcoded sprite tile index, they reserve them dynamically which may end up them using already used sprite tiles, cauing disappearing sprite tiles. Not even NMSTL can fix that.
These were a couple notes when you want to use this sprites. Other then that, I haven't found any issues.
Fixed a major issue with object interaction. The block cluster sprite now resolves "acts like" settings, which makes it a lot easier to work with.
Made the sprite despawn earlier so it doesn't poke in from the top of the screen when it reaches the bottom.