OK so after much testing I have concluded that:
The DLSite Japanese Version does *NOT* work with the mod. However through my escapades I have figured out how to cobble together a working version, however it will take some time and will require a Japanese copy, Steam copy (whatever language) and a rpgmv decrytper/encrypter tool.
The main reason why the game is failing to load images in the english version is because: They don't exist due to the renaming shenanigans from above.
On the Japanese version it's another reason: The encryption key between the steam version and the dlsite version are different. Due to this, the game is failing the checksum required to decrypt the images for displaying during the game proper.
So I tried re-encrypting the Japanese files with the english encryption key and slotted them into the English resource files. Lo and behold, I could load into the school instead of getting a load error.
Now, on to the fun stuff.
So you will need Petschko RPG Maker MV File Decrypter to get this to work. You will have to decrypt the Japanese resources files (Images, Music, etc) and then re-encrypt them using the Steam encryption key. Strangely, the mod also has a system.json and the Chinese encryption key is the same as the English one. Once that's done, you can do the installation part.
After the length re-encryption process you can now:
1) Install the English Steam copy
2) Put in your re-encrypted files resource from a Japanese copy of the game into the English resource folders.
3) Install the mod.
This should work in theory. I say theory as I'm in the preliminary tests to see if all of this holds together in later stages.
The other option is to comb through the json files and rename the original resource files accordingly, or rename them in the jsons.
