1.How exactly do you add custom bust sprites and backgrounds?
I figured out that you add them at editor folders. Also lunch game only after you converted images through converter that lies in folder with same name, otherwise game wont read files properly and won't lunch level.(But as i tried to put custom bust's, converter broke my image totally, idk why)
2.I think you can only replace old ones, since there's no custom chibi sprites folder. OOOR you can put chibi sprites into folder with objects(since sprite of Beelzebub lies there), but idk if it will be animated
3-4. I cant answer, sorry
5. I dont think so, since developer didn't answered on recent topics
And hey, maybe we could bond together to make some mod? I would love to help:D and even if mod tool thing wont work out, i'm planning to learn unity, so we'll think something out. How bout that?