What is a fiduciary?
Fiduciary usually refers to claims fiduciary. This normally applies to self-funded employer groups. The group can either have the insurance company act as the fiduciary (for a fee) or do this themselves. Also in some cases the fiduciary duties are split between medical and ancillary coverages. It's really just a fancy way of saying who administers the claims. But it normally only applies to self-funded groups.
Your company pays for part of your plan (i.e. they are a fiduciary/Trustee on your plan) therefore your plan is not self-funded.
I'm going through the exact same thing right now with UHC Choice Plus. They do have an exclusion written into the policy - however, when medically necessary they can override it. It may take a few appeals though. That said, they tend to deny the vast majority of cases.
If someone tells you to ask for a rider on your employer's policy, they won't do it. It costs a lot for a company to put a rider on a policy (which would exclude Bariatric Surgery from UHC's exclusion list - aka it would make it okay), and most companies can't afford that cost without pushing it onto the employees, which the majority are loathe to do.
I work for a company that deals with insurance and we have a department dedicated to creating and implementing health/dental/vision/disability and life policies for companies and individuals, hence me knowing a lot about the intricacies of this. It's nice to know the details but it sucks when it's against what you are trying to achieve.