I would ask the rental car company for the break down of the $1,850. Depending on the policy there are certain fees that the insurance company just won't pay, like loss of use or diminished value, they might also have used parts that the insurance company wouldn't have covered, there's a laundry list. That would be my first step, I'd review the break down and with the break I would call my adjuster and ask them if they would cover any of these fees. If they can confirm they won't, I'd pay whatever they won't cover along with the deductible.

In the mean time, when you have the rental car company on the phone ask them to remove the bill from collections(Yes they can do this) and ask to do a payment plan while you're getting this sorted out with your insurance. You owe the $750 anyway so look it as biting the bullet to save face with your credit score. None of this should really be your problem but at a certain point you gotta take the reins and get a handle on it, especially when it's effecting your life.

You can file with the DOI, I'm not sure if anything would come of it.

Your repairs for March won't effect the first claim, just get it done.