HI and welcome to the board. I am so sorry you are going through this. Many here can identify with your story and will be along shortly with support and advice.
I have a few questions here. Usually a rehab knows ahead of time how much the insurance will pay. Secondly, why are YOU responsible for anything your insurance doesn't cover? Just because it is your insurance that covers your husband it should not mean YOU are responsible for anything he incurs. Have you called your insurance company to discuss this? My children have had to go the ER when on our insurance but technically we were not responsible for their bills as they were adults. You and your husband have your own separate credit scores. because it is he going to rehab HE should be the one getting the bill, not you. Something to definitely check on! They have to sign papers on this when they enter rehab. I would also try to close out any credit cards with both your names.
Secondly, we as family of addicts learn first and foremost that we cannot control what others do, including the addict. While I truly understand your anger at your in-laws and your husband for the secrecy, the best thing you can do for yourself is realize you cannot control their behavior nor what they choose to do (no matter how sneaky). Right now all you can do is control your life and those of your children. Yes, it is pretty sad the grandmother decided to not acknowledge her grandchildren at Christmas but that will be on her. Who knows the reason?
Do what you can in regards to yourself and keeping yourself safe from any bills incurred from your husband's rehab. It is sad that they could not come together with you to do what was best for your AH. Hopefully he will get the help he needs and find recovery. While he is doing that you can focus on what is best for you and the children. Many have found relief and support from attending nar anon meetings near them. I would encourage you to check it out.