When you create a castle model in the map editor, a file named index.bin with the path to the model appears in the map. HOWEVER, if you put the ORIGINAL index.bin from data.pak into the map, after creating the castle model, it will be written into the ORIGINAL index.bin that you put into the map (meaning it will be an index.bin with all the paths and the path to your model), and then you can put it into any mod in the data folder.
Do you have Discord? Maybe we can collaborate there; it's very convenient for screen sharing. I think it would be easier and faster to understand if you just watch what I'm doing and point out where the error is =)
(Discord - Waterfall#7225)