Hi Christina!
About diabetes and appetite - an uncontrolled diabetic would have a huge appetite because their body isn't turning that food into energy, and all the nutrients get peed out in the abnormally large volume of urine. When first starting insulin, their appetite is still strong, usually because they are trying to regain any weight lost before they were diagnosed, and their system still isn't functioning at 100%. Once treatment starts and they begin to get regulated, their appetite will usually go back to where it was before they got sick.
With Coconut, if the cancers were screwing up his ability to properly digest his food and convert it to something useful (energy), then although he might have appeared to be eating okay, he wouldn't be getting the benefit of eating that he would had he not been sick. He wouldn't have been getting the BG boost a healthier cat got, so the insulin would have been too much, pushing his BG down, and causing hypos.
Chocolate - since she is used to 5u, then you will almost definitely see a high number in the AM, so expect that. I am so happy that whatever instinct or whatever caused you to check her tonight pre-shot was there for you. Humulin is a fast acting, shorter duration insulin, and shooting 5u into an 83 could have gone really bad, really quickly.
As soon as your vet opens in the morning, maybe call them and discuss dosage and make sure to tell them she was at 83 tonight. Make sure and get an AMPS before talking to them too. If she's lower than 200, my guess is they'll tell you to cut her normal dose way back.
Carl in SC