Here are a couple articles for you:
Peculiarities of felinehyperadrenocorticism Update on diagnosis and treatment by Lara Boland and Vanessa Bars (2017). Quote in there, page 940 Trilostane dosing is 30-40 mg per 12 or 24 hours, though more recently starting with 1-2 mg/kg of cat. At 4.2 kg, that would be 4.2-8.4 mg. Sounds like the 7.5 would be OK as cats have done much higher.
This 2021 article:
Feline comorbidities: Recognition, diagnosis and management of the cushingoid diabetic by Audrey Cook and Jeremy Evans mentions starting dose of 10 mg twice a day (see section on treatment). There is an interesting paragraph referring to dosing for cats with diabetes, that suggests twice a day dosing is best:
I read another article saying that lowering the blood sugar values (and insulin dose) are indicators that the excess cortisol is being controlled. Have Evey's insulin dose lowered since starting Trilostane? If not, maybe that is a sign she needs an increase?