You're very welcome, I wish I could do more to help.
It's good that the father is calmer now and the mother has been feeding the chick. Some females are exceptionally good mommas and are able to raise their chicks till they gain their independence, and it seems you have been blessed with the perfect budgie mother.
It's also perfectly normal for the mother to not sleep on the nest for the night. When chicks get mostly feathered and are able to control their body heat, the mother will no longer stay on the nest. Some chicks even protest and want their mommas out of the nest so that they can all sleep with the extra space on the nest.
As to the food, you can offer easy to dehusk seeds like spray millet, canary seed, flax seed and Niger seed. Egg food is also a must, not only is it highly nutritious, but also soft and easy for chicks to eat. You can also offer soft veggies for the chick to try out, soft spinach leaves are a good starter veggie.
I'm not familiar with avigold, so can't offer my input on it.
Guardian Angel can be helpful as it will give a good boost on your budgie's currently weakened immune system.