Well there is your problem with testing. Treats are the key. But just like all food is not created equally neither are treats. Anything pure protein works. Now with my tribe I don't use commercial made. But instead both Autumn and Cassanova get plain boiled chicken breast, water packed tuna or sardines even bites of low fat cream cheese. Now we're not talking a pile but bites about the size of a piece of kitty kibble and 2-3 of those.
Next is attitude, a cat to cat language is 10% vocal and 90% body. They read us like a book. If we're stressed, they're stressed. If we're happy and confident then they are happy and confident. We can smile and talk sweet to them all day long but if that smile stops at our lips and doesn't reach our eyes they know we're faking it.
When you have multiples its like having a house full of 2 year olds. If one is sick they're grumpy and their siblings irrate them. And their siblings will pick on them. Usually not when mom is watching. Just like human children you don't pull your sister's hair in front of mom you do it when her back is turned. Now because a cat's language is mostly body language you may not even be aware of it without watching closely.
I have one who we affectionately call the Sh*tten. He's a sweet guy but mentally he's stuck in kitten mode. He is the true obnoxious little brother to my older cats. He won't attack them outright but he'll pull the "I'm not touching you " game just like a little kid until they get fed up and run then the attack is on like Donkey Kong. Or he'll lay and stare at them until he gets the same results, they get fed up and run and he chases. To him its play to them its an attack and they hate him. Maxwell has had so much of him that he'll growl if Myrrdin even walks in the same room. But Maxwell is very submissive and doesn't fight back. Myrrdin tried it with Autumn but she's a retired barn cat, she tolerants no bad behaviour, she knocked the stuffing out of Myrrdin and it never happened again.
The biggest mistep I see people take with cats is stepping to stop squabbles. As long as no is drawing blood or removing flesh let them work it out among themselves. Cats have a pecking order. Cats are also matriarchal and Queens are territorial. When Calley was very ill she lost her rank and possibly in subtle ways Buddy tried to raise his so now that she's feeling better she's trying to put him back in his place.
Or at least that's one thing to consider. The other is that she just doesn't do well personality wise with Lantus because of its acidic base and would do better with a neural base insulin like Levemir or Prozinc or even BCP PZI Autumn had stellar numbers on Lantus her personality sucked on it , she was vicious on Lantus and a purring mass of love on Levemir.
Mel and The Fur Gang