Don't say anything, You're on the right path.  It's stuff DH, PS, TT and FF are all going to have to work out, or not work out.  Keep educating yourself, stay detached.  I'd postpone the Room discussion til things cool down.

Do not re-engage! Hold strong. There is nothing ahead of you but "heart ache" if you get involved in any of this. Stop researching anything about her medications or symptoms - no good can come from you getting involved in any way! Any suggestion you make or info you give can easily be turned around and used against you.

I know it is your nature to want to help, I am the same way. But all of these people, including your DH, have shown you time and again who they really are and what they are capable of - and it never, ever turns out well for you. Stay clear of all of this.

You know that you cannot get sucked into this.. you can't fix any of it.  Your SD  has to deall with what she is dealing with as a rapidly approaching adulthood human being.  Your husband needs to deal with his daughter's issues.. and while you don't have to pile on grief.. you can't walk this path for him.

and remember.. your SD has shown some pretty big biting the hand that feeds you episodes.. so you need to be calm and backed off. 

STOP. FULL STOP.

You're not a doctor and she was seen by professionals. You don't need to get all "Matlock" or "Columbo" and suss out the problem. Not your circus. Not your monkey.

The room? That can wait. She has another year of high school. No discussion about what to do with her room is going to be well received. Take that from a bio mom. If I had a spouse bugging me about repurposing my bio child's room while she's still in high school, I'd be PISSED.

The best use of your time at this point is to decide what you will and will not accept, and how you will respond if one of your boundaries is violated.

I have been you - trying to control everything. The only think you can control is yourself.

This resonates with me. It's hard to stay detached when you've previously loved and been close to your SD. It's like being in that space when you're deciding to leave a marriage. You have these little glimmers of hope that your spouse will turn back into the person they were (or you thought they were) when you married them. 
If you go the way if researching, maybe you can just give any useful information to your DH, like you would if he needed help understanding a friend or colleagues situation. But try not to risk getting your heart stomped on. 

Everybody is giving you good advice which I agree with.

But, I was thinking about PS, not that YOU can do anything about it.  If I were an obese teen going back to high school, from a divorced family with a highly dysfunctional mother and possibly dangerous sister living with me, I might have palpitations, too.

Not all heart palpitations are created equally.  The EKG wave form on the heart monitor tape (I am not a doctor, but I have had a number of Dx heart monitoring events) determins if it is an event, or little to nothing.

My first was when my heart started skipping a beat. We had just gone to bed for the night.  Lying in bed I could feel my heart stop, the restart. I did not feel bad, it was just different. So, I drove myself to the ER, they put me on a heart monitor, told me to not drink so much caffein, and sent me home and advised that I follow up with my PCP.  My PCP referred me to a cardiologist. I did a nuclear cardio stress test, they found an RBB (Right Bundlebranch Block).  Excercise induced. Basically the trailing edge of my heart electrical signal wave form drops off slowly rather than sharply as it would in a normal person.  RBB is a reasonably common condition in the general population and ... is inconsequential according to my cardiologist. 

I have had a couple of other nuclear stress tests, no issues beyond the RBB.

What did the Docs say about the EKG or Dx results on PS's arythmia?

I would stay with the new CLove model.  No PS moving back in, empty your new meditation room on her B-day or graduation day, whichever ends the CO, and keep the Harpy trio the hell out of your life. Daddy can deal with with them from a distance. I would make it clear that DH is YOUR DH not TT's her tires are her problem, the SDs are adults and it is not his job to cater to them, solver their problems, etc... 

So many parents abandon the position that kids interface with parents as the parents stipulate, not the other way around. IMHO, when that model shifts, the kids can suffer the consequences of alienating the parent.  No money, no support, just "I am so sorry to hear that.  Let me know how it turns out. We are going hiking. Buh-bye"

Click!

Stay out of it. Let your DH handle this medical issue with HIS kid.

Don't get sucked back in. I've done that before....multiple times with the BM when she'd "attempt suicide" with options that would never have worked anyway. Think me rushing across the parish (county) to BMs house, being called by her neighbors and police, only to drive up and find her in the ambulance, laid back, talking on the phone and laughing. SMH And every damn time I jumped back in, it backfired on me.

I'm not saying your SD doesn't have medical issues but even if she does, she's not your responsibility. 

This actually brought back memories of a former co-worker who regularly complained about heart problemw and who saw a variety of cardiologists on a regular basis. None of them ever found a cause despite numerous exams and tests. All her problems miraculously disappeared within two weeks of her joining a hiking group (we'll say nothing about her wearing heels to hike nor about the umbrella ...). Funny that.

Detachment and indifference is the key to not becoming the one who gets sick.  Sounds like you have A LOT of stressors.  Don't get sucked in and stick to your new way of dealing.  Remember- you didn't create this and it isn't your situation to fix.  Unfortunalty, as step parents- we have to deal with the damage caused by the BM, lack of accountability by the DH and the drama/hostility etc by the stepkids.  Take care of YOU and for nice people we tend to think of others and want to help.  Re-program the way you think at home.  I'm so sorry.  

The heart palpations are probably due to the stress.  Mine came from long covid- had covid in 2020- was on oxygen for a year (was fit, used to mountain bike, hike etc prior to being sick) then the heart palpations came on with a lot of other issues.  After a year of physical and occupational therapy- keeping journals of when I got crashes and got very sick....it was all during times that my SD was visiting.  Don't let the stress get to you too.   Take care of you and use some of your kindness on yourself. 

Maybe I missed it. Has she had an echo done of her heart? THAT will rule out or confirm a bunch of things. Could be Mitral Valve prolapse (MVP)--usually feels like flutters or skips, than tha- thunk, back into normal rhythm. 

Not suggesting you should take her to the cardiologist. OR dive head first into any of this. 

But for real....get the echo done.   

Part of the practice of detachment is training ourselves not to kneejerk, not to react. I had to acknowledge that my initial impulses to Do, Fix, Contribute in some way were often wrong and instead cultivate doing nothing. Doing nothing is still something, CLove, and can be just as powerful as doing something. 

When you think about it, most step poo belongs to others and there's seldom any urgently required reaction from us. Sitting on our faulty impulses - saying nothing, taking a pause and breathing, just being a disinterested observer - these are all skills we can hone and keep in our disengagement toolbox.