In vanilla, Grass and Fern can be replaced directly with the block that is in your hand rather than forcing the block to be placed next to those plants.

For Alex's Caves, the following blocks I feel should be made replaceable: Bone Worms, Underweed, Tree Star, Fiddlehead, Curly Fern, and Pewen Pines.

This can be done by adding .isReplaceable() to the item property builder in the item's constructors.

(Also, is it intended that Bone Worms can be placed outside of water on land in Creative?)

I'll concede on the Bone Worms, underweed, and pewen pines

but the curly fern, tree star and fiddle head have important uses that could make this an annoying feature.