Please help with INSANE surprise bill!!

A few years ago, my mom was in a car accident and was taken to a hospital in an air ambulance. We received bills, and I always sent them to insurance. We hadn’t heard anything in years and assumed all was taken care of. However, we just received a bill in the mail stating that the air ambulance company sent the debt to a collection agency. The bill is originally for ~$33k, but they’re offering ~$19k if we pay lump sum. We have since changed insurance. We live in Tennessee, but I’ll have to check where the accident happened. If needed, I can edit this post with other pertinent information later.

My questions are:

What should be the first course of action? Do we send this to our new insurance, and let them determine if this is a legitimate obligation to pay a debt?

If the debt is legitimate, should we let insurance do all the talking, or since this happened before our coverage with them, will they take no part in discussions?

Would it be foolish to contact this collection agency without a lawyer?

Could we negotiate low monthly payments, or would declaring bankruptcy before anything else can happen be the best course?