RE: challenging the mental health claim limitations
If your employer is completely self-insured (often utilizing an insurance co to administer benefits), federal laws/regulations govern the policy.
If your employer is not self-insured (hires in an insurer via insurance policies), state laws/regulations govern the policy.
Most State Banking and Insurance Commissioners have a list of insurance companies/employers and can tell you which category your employer falls into and whether state or federal regulations prevail. These offices can usually give you more info you may find helpful.
Some State Banking and Insurance offices also have an advocacy service which may offer to help you in your desire to challenge the terms of the policy.
A side note: if your stated illness (disability) caused you to miss a deadline, it seems that shores up the disability claim (shows impairment), rather than closing the claim. An attorney would try to argue this I'd think.
Hope you find some piece of info somehow helpful.
Fighting for LTD claims can be very challenging and can be won.

WC