Entity Replace Blocks in Structures

So this may be slightly complicated and I'm unsure if there is a way to do this even, but, I want to make it so that when a Vanilla structures such as the stronghold or a spawner dungeon generates that there is a chance for one of my custom entities to replace a block in that specific structure.

I am trying to make a mimic entity that mimics multiple different items/blocks.

Similarly, I also want to make it so that when that entity dies, it sets a block on the ground with the same NBT data/inventory that the entity had.

Any ideas on how either, or both, of these are done?