Are prepaid dental plans covered under COBRA?
The plan has no charge for preventive and cavities and set dollar fees that the member pays to the participating dentist for other nonpreventive procedures

An employee& dependents can elect COBRA for dental plans with medical or as a stand alone. They would pay the premium subject to 10% administrative fee permitted. Copayments have nothing to do whether COBRAble or not.

I thought the admin fee was 2%?
Does it matter that the dental plan is 100% employee pay all plan and perhaps not subject to ERISA?

Yes, the COBRA admin fee is a maximum of 2%.

Is the dental plan sponsored by an employer, who is otherwise subject to COBRA? Or does the employer just allow someone to come in and sell the product, without endorsing it? Is there any mention of continuation in the plan booklet?

Employer sponsors it and marketed to me as part of teir benefit package with my offer letter
I never got an ERISA-type SPD ; I did get a booklet provided by Dentaquest which details what's covered, the set fees the employee pays, etc... but no mention of continutiaon coverage. I did verify that the dentla plan is an HMO.
But then , I never got a health plan SPD but did get a COBRA letter from my employer to continue the medical plan with another carrier

1 key thing I neglected to mention is that the voluntary dental benefit is run through a 125 plan
I would think this fact alone would mean it is employer sponsored?
Yes, my former employer easily met the 20 employe threshold to be subject to COBRA

It sounds like you have a scheduled dental plan, which sets a reimbursement level for just about every covered dental procedure. This plan would not be treated any differently than a passive PPO plan or an indemnity plan. Therefore, it should be subject to COBRA.