Principal Financial Group dental insurance nightmare. Someone please help me.

Two months ago I went in for a routine dental checkup, and after a bunch of poking and prodding, my dentist informed me that I had moderate periodontal disease that required a scaling and root planing procedure due to the bone detachment, severe pitting, and oral bleeding present (yeah, sorry it's not a pleasant topic)

My insurance provider (Principal Financial Group) specifically outlines this exact procedure in our benefits handbook as a "unit 2 - basic procedure" that is covered 80% once every 24 months.

The insurance company denied to pay any amount of the procedure because they determined that the procedure "wasn't a dental necessity."

My dentist (who happens to be a well trusted family friend of over 30 years) appealed the decision and provided x-rays, periodontal charts, and notes outlining the obvious need for the procedure.

The insurance company denied the appeal, and then denied the second voluntary appeal, indicating that based on the evidence provided "periodontal disease has not been demonstrated based on pocket depth and loss of attachment". The appeal that they were responding to, from my dentist, stated:

This letter is in reference to a rejected dental insurance claim [omitted] for a Mr. [omitted]. He was treated in our office for moderate periodontal disease with scaling and root planing. The criteria for periodontal disease is periodontal pocketing in excess of 3mm, a loss of gingival attachment, and bone loss around teeth. His periodontal chart is enclosed. Mr. [omitted] presented with multiple pocket depths of up to 5mm, heavy bleeding upon probing, generalized loss of gingival attachment, and generalized moderate bone loss which is readily apparent on the enclosed copy of his radiographs. As he meets and exceeds all of the criteria for periodontal disease, I strongly request that you reconsider his valid claim for payment of services rendered.

When I contacted the insurance company with all of this information, their response was "the benefits handbook provided to your employer is just a guideline of your benefits. We use a number of determining factors to approve or deny any procedures."

When I asked them to point out to me where this was outlined in the documentation provided to me, they stated "if you want the specific guidelines used to determine eligibility for benefits, you must request them from us by mail"

My dentist highly advised me to contact the American Dental Association to file a complaint, which I did, but the ADA said they will not get involved in a dispute between an insurance company and a patient.

My question here is, what are my options at this point short of getting a private attorney, which I obviously can't afford? The insurance company is refusing to pay for a procedure that has been explicitly outlined in my benefits package as a covered procedure.

TL;DR - Insurance company is refusing to pay for a covered procedure. I can not get any straight answers as to why, and nobody is willing to hold them accountable. What are the options for someone who can't afford an attorney? Is there any proven way of holding insurance companies accountable when they are clearly operating outside of their communicated guidelines?

*edit: I am located in Michigan, USA.