I used to know some people who loved to snort klonopin. It was before I had kids, I worked with them. I didn't know anything about benzos at that time, just that they seemed to love to crush and snort them.
I think you need to figure out if this marriage is worth what it is costing you. I know it has made things tough with your biological children because they don't want to be around the house. You don't enjoy being there either from what I can see. She went to rehab without telling you and then texted an accusation that you are having an affair when there is no way for you to reply to defend yourself? Honestly, has she ever given you any way to defend yourself from her accusations or in the untenable situation with her spoiled addicted son? If she wanted him insured, she could have gone and purchased insurance for him without making you do it. She preferred to make your life miserable than to deal with the situation or see the lack of insurance as a logical consequence.
She simply won't change. She went to rehab for her own reasons, but chances are that she will NOT change anything with her relationship with her son. She certainly will NEVER support any measures that you take to make him accountable. Once she has been to rehab, she can use this to say that she is working on her problems and you are not doing anything. This means that all the problems are your fault. I have been down this rabbit hole and seen others go down it. You can accept the situation as it is, accept that if you want a life with her in it then you will have to live this way.
Your other choice is to separate from her. It might mean some financial losses, but what will that mean if you are happy? If you are not dealing with stress related illness? I sense you are a man who makes decisions and follows through with them, so this waffling and never making a decision or following through has been incredibly hard on you. It has probably been harder on you to have her waver back and forth than it would be to have her say that her son was always going to be this way and was always going to live with you. I don't know that you would accept that, but it would be easier to live with than the push/pull thing she has going on.
As the situation stands, you may have more money for now, but something the son does is going to create a huge liability and it will cost you a fortune to clean it up. Or you will go out of town for a weekend or a trip with your wife and he will have a party. He will post it on social media and the whole town will show up. It will destroy your house from top to bottom and cost you tens of thousands of dollars to repair or replace everything. If not more than that. Plus you will have to deal with criminal and even civil charges from what the stupid partiers do at your house when you didn't even invite them!! You left the irresponsible stepson there, and he had the party, so as the homeowner, you get the bill. Or his mom will insist on paying for top dollar legal defense if someone gets hurt or whatever. Lots depends on the laws in your area, of course.
It is something to think about as you decide if you want to continue on in this relationship. You are obviously a very nice man. You raised children who successfully launched into the world with good careers and lives. Then you got into this relationship and I am not sure you know how to cope. I know most people wouldn't. It is okay to admit that this just isn't working for you. You stuck this out WAY longer than most people would. You have really gone above and beyond to try to help both your wife and her son with their problems. You have spent time, effort, an enormous amount of money (from what I can gather) and invested a lot of yourself into trying to help both of these people. I am sure you have learned a lot from these relationships, but I don't think you are getting your needs met in either relationship.
All relationships are 2 way streets. Sometimes our kids take more than they give, but especially as adults it needs to be a 2 way street. You seem to have tried to communicate your needs to your wife. I don't think she is capable of either hearing your needs or meeting them at this time. Her addiction and her codependence with her son are both too great to allow her to have any resources left to spare for you. It seems to me that you are becoming tired and depleted from giving and rarely receiving in these relationships. At some point you have to protect yourself and say enough it enough. If you cannot bring yourself to do this for yourself, do this for your own children. They deserve to spend holidays and vacations with the father they love. From what you have said about them, they want the best for you. They want to see you happy. They love you.
I would caution you to do whatever is needed to protect yourself in the event you choose to leave your marriage. Seek advice from an attorney to learn what to do to protect your assets. Make sure you have documentation of what you have spent on everything including each of your children since the marriage. If she is accusing you of something, be sure you can defend yourself well. I am not saying to try to hurt her in the settlement, but be sure to protect your interests. Also be aware that this is the most physically dangerous time in a marriage. Never underestimate the danger. If you feel you are in any sort of danger, trust those instincts (no matter who tries to ridicule you) and get away from the situation if you can. Don't let her goad you into losing your temper to the point that you lash out in anger either. I know several divorce attorneys who suggest that clients get their spouses to lose their tempers and then catch this on tape or video to be used in court. Be aware that it can happen in this type of situation.
It is good to hear from you again. I am sorry that things have not gotten better. Welcome back!