I think it can be possible to make custom blocks that can fall, like other various blocks like sand. Also, can it be possible to have custom hardness values or custom breaking sounds, though it might not be possible to have custom hardness values. Does the Unbreaking enchantment affect the custom durability? Just a honest question that I have.

1. "Custom Blocks" as in actual blocks are not really possible, so creating an API for these features would be somewhat difficult. In 1.14 I could see a feature to convert any block into falling sand, though it would be up to the user to determine when the block should fall. However, this really wouldn't work well with blocks using armor stands for custom models (or other entity markers).
   Custom breaking sounds are possible for the end user to add, but and API doesn't make sense. If you have your block marked with an entity, then when the entity detects the broken block, have it stop sound the original break sound and play the new one. This may also be doable with block loot table in 1,14.

2. Yes, unbreaking effects it. However, mending does not (at least not right now).