Keep interacting with your DD, as much as possible, in the same vein as before DSS arrived. Drop the guilt on praising her in DSS earshot, even tho she is only 7 and your 12 year old DSS isn't as good at doing the same things. It won't harm the boy to hear another praised! This change should ease your feeling of being an emotional wreck a little (been there with DSS myself, know what you mean).
You've done a Good Thing taking in your husband's son, the lifelong half sibling to your daughter.
The two kids are different. I don't like labels but my DSS was happily labelled as different, and still is by birth parents and siblings at 19.
So here, me being a bit guilty of projecting my own experience, I can tell you how I see your situation.
Your boy sounds anxious (and shy with social interactions?), but loyal to friends, and with a sense of right and wrong, but he is driven by fear. I wonder if he has black and white thinking, is very self protective, and doesn't see things from others point of view (he smelly if he wees himself, he makes others tired if his alarm wakes them, loses trust if he lies). Your DSS hates confrontation and goes into internal panic.
With maturity these traits could completely be ironed out. He might be 'different' for years. It might be your DSS will have emotional struggles always, after a very difficult start in life. This will be a heartache even more so for DH. Whatever personality your DSS has, he is unique, he is who he is and can make a great contribution to society now and as an adult. He is full of potential and he is in a good place with family now.
You must be shattered. Emotionally and through lack of sleep, and you feel the worries of the world have dropped into yourr previously smoothe running life. It seems you and hubby need to reform as a team, and talk, smile, cuddle and have some fun.
You need to shift the emotional burden and decision of parenting style and medical help needed, onto your DH. He might be happier feeling he is doing something and admired for it. And then you can wholeheartedly support DH.
I too, lean away from the heavy chats and try far harder just to naturalise casual conversation full of loving support and occasional gentle chastisement, or emotional grounding for my DSS who, I describe it as, can't see 360 degrees. He just has his one angle. However, he is capable of learning if I point things out gently. Any poor tone from anyone leads him to anger (or occasionally tears).
Go gently on yourself and try to absorb you are doing your best. Your husband needs to know you don't judge the boy or hubby as a failure. Tell DH youre happy but tired and let DH gift you sleep/tea out with DD/ time off. Admire your husband's good traits out loud to him and allow DH to be the stronger, more involved parent.
You sound a fab step mum. Get some renewed energy.. Try to give yourself a week long "emotional-holiday" of not fretting so so much. Just Be. Live in the moment.
Treat DD to some mum time.
Gawd knows how you solve the money issues? Get a Saturday job in a nice calm adult only environment (hotel/spa/estate agency??)??
Apologies this is long. Your experience touched me.