You are an amazing Mother Gigi! I know you have been going through a rough patch with Bilbs lately, especially with the food.
Have you tried augmenting some of the "sometimes" foods she will eat? Making savoury muffins? Or sweet muffins with a healthier spin - like some vegan recipes, carrot cakes sweetened with apple puree, etc? You can use puree pumpkin or sweet potato to flavour them too, you can use wholewheat flours or nut meals in place of flour, and healthier oils in place of butter.
It isn't ideal, but for the time being it may help ease the tension a bit and give you some breathing room to take on some of her other challenging traits at the moment, itms.
Perhaps some more obscure foods that may look or taste like things she already knows and likes. Tofu, exotic fruits, home made chips from yams or sweet potatoes.
Do you have any space for growing your own veggies? Not masses, but a pot for tomatoes, or strawberries, zucchinis are easy to grow also. Perhaps having that connection with her food may help.
Will Bilby eat if she is at someone else's house, or someone else has prepared the food for her?

I don't have any ideas about parenting programmes or anything, sorry, there are some FB pages though like creative Parenting that may be able to help you with that. Maybe the PET book could help you? Or Positive Parenting.

Anyway, I hope you can find some answers and ideas that will work for you both and move past this to happier times. xxx