"Defacating and urinating on herself, lying, extreme attention-seeking behaviors" 

Doesn't sound like an "amazing" child prodigy to me. Sounds like a nightmare. That is all.

about your SD.  And she most assuredly has done something wrong or SHE would not have built your revulsion to her.

Stop deifying her as not having fault for her choices and behaviors. At 8, she is old enough to know right from wrong and to be held accountable for her choices. Increasingly so going forward.

Nothing is more nauseating to people than parents that try to pawn their crap behaved kids off as something special.  You do not own her behaviors so quit making excuses for her crap behavior.  Your SO is markedly absent of mention in your original post. Hmmmmm?

Yes. She is a child.  But she is not faultless in this.

How about Doctors and therapists?  What do they have to say?

IMHO of course.

And speech delays. They don't urinate or poop on themselves. They go to toilet and know when they have their urges for either. 
 

we read too often of messed up kids not happy about a divorce who intentionally pee/poop where they sit intentionally just to piss you off and hope you leave their parent. 
 

this needs to get addressed as its not normal.

Something is wrong with your SD, but your SD is 8 and can't fix it on her own.

Something is causing her to urinate and deficate on herself. Something is causing her to lie. Something is causing the attention-seeking behavior. You seem to recognize this, and you seem to acknowledge that it is her parents' fault.

Now what you have to do is shift the resentment you've built toward her and direct it at your partner for doing jack all of nothing about it. It's really easy to blame the SK for our pain amd resentment because they aren't the person we love and want to have a relationship with. Same as it's easy for SKs to resent us for pain and anger they feel toward their parents and the divorce.

If your partner hasn't stepped up to fix the problems with their daughter, then you need to put your foot up their backside (figuratively) and tell them that they're hurting their daughter in the long term. My DH waited too long. His hands were partially tied, but he let his own fear also get in the way. While things have worked out (mostly) now, my SKs have a lot of damage to undo. My DH has massively stepped up and swallowed his guilt for their sakes, but a lot of damage is done.

If your partner won't step up, you need to fully step out of this relationship. You can't hide resentment as well as you think you can. Your SD will know, eventually. You'll crack, eventually. You'll be beyond miserable, eventually. You either have to sell your soul to accept a situation you don't accept OR you become a hateful person who can't accept it and can't deal with it.

IF your partner has stepped up and SD just has issues - like having autism or a mental health disorder - some of this may be par for the course until the right treatment and routine is found, and that can take years to find plus have to be changed and adapted as she grows older. You may not have the energy to deal with it, and you don't have to. As much as society thinks SPs should sacrifice everything for their SKs, they shouldn't. They can't. The reward isn't the same as it is for BPs, but the losses can definitely be worse. Your SD won't benefit from having you around if this isn't what you want or can handle, and it doesn't make you a bad person for not being able to handle it. Many BPs can't handle it, but they are forced to find a way to. Antidepressants, alcohol/drugs, side pieces, eating and exercise disorders - you'll find it among parents dealing with exceptionally difficult health diagnoses. Not every parent, of course, but many. You aren't the parent, and you don't have to go down that path. You are allowed to save yourself.

Finally, the symptoms you describe sound like they could come from some form of abuse, particularly sexual abuse. If you have any reason to believe that may be happening or know it has happened and could be continuing, sound the alarms. You can't get SD help for her issues, but if you know a problem exists, you can let DCS/CPS step in and take control. Don't let abuse go because you're asked to ignore it, or turn a blind eye because what you see couldn't possibly be what is going on. Sometimes the only way we as SPs can help a kid is by nuking our own relationships and letting people with authority come to their rescue. 

Your SO is doing nothing.  He's not a good parent.  This kid needs help.  Your in for a world of pain.  No one can like a kid like her,  remember. Something can happen to BM.  Or BM finds a boy toy and leaves. You will have SD 24//7/365 

This kid is 8 and is crying out for attention. There are so many red flags that I don't know where to start. Please get her into therapy ASAP. She needs help NOW. 

That she is soiling herself is a major, major, major red flag. Why on earth are YOU taking on the job of finding therapy for her, let alone attempting to live with her under your roof? She has parents. That's what they're for.

I'd have been out of there last week.