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.
Yoshi's Island originally does not have support for 6 keys because the act of swallowing/obtaining an egg/item while your egg inventory is full results in the very last item in your egg inventory being discarded even if that item is a key. Effectively, you can lose a key, which should NEVER be the case.
This patch prioritizes and keeps keys even in the case of full egg inventory.