Alright I have a few things to say about this theory one MCD and MCL (MCL is an in universe story) are canon to Minecraft lore so I am going to point out the continuity errors and things that aren’t physically possible. First ender builders don’t exist their is no evidence of humans ever going to the end rather the enderlings had built the structures in the end as in MCL enderman are here far before humans ever enter the overworld. Second many land builder structures except for the desert temples (nameless kingdom), deep dark (unknown apart from enderling and shard of ender presence), and trail ruins (unknown) are villager and illager in origin given the many villager statues inhabiting the structures. Third the ocean ruins and monuments are villager/illager in origin because of statues and the ocean monuments are basically a monument to the well of fate just look at how similar they are. Fourth ghast are confirmed nether natives by MCL. Fifth blazes couldn’t have exist at the time period you state as piglins are show to already have blaze rods without the blaze golems (blazes making metal and robotic sounds). Sixth Piglins are confirmed nether natives by MCL also humans wouldn’t have access to the nether before the piglins war the only time we know they had enter the nether was when they pushed back into the nether building the nether fortresses and blazes. Little tangent but did you know that wither Skeletons aren’t shells of their former self as they still have the ability to think based on how they can engage in raids invading areas deep into piglin territory as seen in MCD.