oh dear, quite scary to wake to that! poor you and Gizmo. Arthur had a lot of the same beginning symptoms and problems as you describe so I can sympathize.
Firstly, I would like to congratulate you because already you have wonderful dedication and great application to the task of getting Gizmo on track, despite your stress and the ocean of sometimes conflicting info. Well done.
Secondly, I can advise at this point, the aim to to get Gizmo stable first, and we can push the longterm worries aside for now.
Keep in mind during this first period of adjustment that Gizmo will experience big changes in appetite and energy as her wee body slowly becomes accustomed to the hormone. we all know how our hormones affect us monthly, and Gizmo receiving insulin may feel nausea, a stomach that feels full after only a small feed, a bit of bloating and discomfort... Gastroparesis and pancreatitis are common among diabetics, but most often ease with good regulation.
I would advise sticking to one safe food and not swapping and experimenting - it is not the food that will create the appetite, but the regulation of nausea or discomfort. while you are trying to match the insulin dose to the food, it is more effective to do so by Not swapping foods with different calories and ingredients. Broth or even adding warm water to more solid wet food is a great incentive.
In the beginning, I gave Arthur 6 to 8 tiny meals a day because the supposed 2 feeds, 2 shots just did not get him settled, and was too risky if he then vomited after a shot. I had tried that schedule advised by the vet for probably two weeks with Caninsulin, and it was just a nightmare. Regular small feeds meant he was safer and felt less queasy. It took me 6months to convince a vet to give Arthur a script for Lantus (another saga).
Btw, the covering of the remaining food is done to protect it from predators. By covering the meal with soil or foliage, the scent would be masked until the cat comes back later to feed again. It is a cat fridge, if you will

This is good news because it shows Gizmo approves of that food, but just does have the appetite or stomach space to eat it.
I also used the sachets of probiotics drizzled over food in the beginning to try and help digestion.
Importantly, if Gizmo is having both vomiting and diarrhea, dehydration will be an issue. I took Arthur to the vet during those epidoses for subcutaneous fluid. A small animal just cant keep up with water loss in that situation and I have to say, the fluid injections really work and gave both him and me such relief. It was an important stepping stone to recovery.
I can also recommend the tiny little paste posts GimCat Pate Deluxe in Veal to raise calories after vomiting, and be safer if insulin was already given prior to vomiting. I have also spread it on food to get him to eat.
I hope today is a better day for you! All the best!