There's a whole discord for this game where we frequently discuss modding problems and stuff like this. You should join and ask for some help, I'm sure we can steer you in the right direction! A lot of this is probably just kinda down to the game's behavior not being well documented; like, I don't think the side sprites actually have a backhair layer. And, I don't think your files actually need to have an ".import" file with them to be loaded into the game as a mod. Again, join the Discord, and we can help you work through it, we always love to see more mods and modders!!
You are misunderstanding my statement there, I said I rename it according to the .import file but I don't use Import File because that wasn't necessary.
And for the SIDE SPRITE backhair layer, there is a file for that in the core/root of the APK, but I don't think it works the same as in mod section.
Honestly, I didn't use discord very much because I lost my account with phone number on it. That's why I post the problems here, hoping that the dev notice the modding section doesn't work the same as stated in the general modding guide.