I don't have the experience you all do. Mine comes from a bro who was addicted, primaril. I do recognize that it is different.
If I am rememering things correctly, this is the first time she has called reha and gotten herself there, so that is a big difference from other times. in my opinion it is real progress. Given that, I think that finding a way to support her that you are comfortable with and that does NOT involve her living in your home, is reasonable IF IF IF it feels right to you. Each situation is individual, and each family also is. You have to do what you can live with, period.
I do think you need to figure out when you will find it reasonable to insist she support herself, when in your mind she will be ready to handle these issue herself. Much of what she tells you seems to be designed to either get you to give her what she wants or to be mean to you. Hopefully that will change with sobriety and learning to deal with her problems. But at some point all of this, her living situation, her $$, etc... needs to rest on her shoulders. We can all tell you what we did at that age, how many kids we had, homes we had, etc.... but NONE of that matters. What matters is how and when she will take responsibility fo rher own life. It is my understanding that this is a big part of rehab and the 12 steps. So at some point, this needs to be a goal for her. Is she capable of handling her own life? Is this something you have to work toward in baby steps? I can't answer those questions for you = you, husband, difficult child and a counselor need to discuss this. Preferably you and husband should discuss it with a counselor before you discuss it with her.
I do have one idea about the gas money. Figure out what you are willing to spend on her gas. Get a gas card and let her know that you will NOT give it to her. That you will ONLY pay for gas, and that you will NOT give her the card. If she goes over the amt you have on the card in the time that you are willing to pay for her gas, then she will have to figure out how to get more gas with-o help from you. It will help her be mindful of the cost of gas and how much she is driving, along with how much it costs to drive. Let her know the $$ amt on the card, and let her know that it is YOUR card to use to buy gas for her, and her budget is the amt on the card and there will be NO more gas money when she has spent all of it. NONE. Then stick to that limit.
If she just has you pay for gas, the $$$ sn't real to her nor is any limit.
Whatever works for you, and is what you feel you can do to help her, is what needs to happen. Part of me feels very much the way Dazed does. I know that it sn't the right thing to do at every time and in every situation though. I think at some point you will have to draw a line and say that she has to go and support herself and live her own live with you as part of it but NOT with you paying her bills, but that will be when YOU and husband are ready and you feel it is the right time.
I am sorry it is such a difficult situation and so full of conflama.