You are in the very early learning stage and this all seems so overwhelming. Of course you want to be able to rely on your vet but vets are only human and IMHO have one of the hardest jobs on the planet. Your vet learned about all sorts of different diseases for numerous species in vet school. How could they possibly be experts in everything? Vets do their best but they can't be expected to know it all. They have patients who can't tell them if it hurts or when or where it hurts and often times hide their illness very well. When you get right down to it, vets have a very tough job and do a excellent job most of the time despite their inability to clearly communicate directly with their patients. Human doctors are only dealing with one species but we still have specialists for everything.
Everyone helping here has dealt with feline diabetes themselves and has learned from their own experience and the experience of other members here. You will soon be far more of an expert with your cat's diabetes than your vet can ever hope to be based on their training alone. Hands on experience is the greatest teacher and unless your vet has a diabetic cat of their own, they are never going to get that hands on experience.
You are the vet's client as much as Salem is, and as Salem's caregiver, you have to be comfortable with the treatment plan. If your vet isn't willing to listen, seriously consider your concerns, and hopefully learn, there's nothing you can do to change that. That said, you are in control of what you do for Salem at home. This has nothing to do with authority. Your vet should work with you as your partner not as a dictator.