Oh man, I just read through this thread and also am in the same boat as you. Well, not the same, but close enough that I can completely relate. My niece was 6 when she moved in with us, her brother 2.5. Neither is diagnosed with attachment issues, though I'm pretty sure that he will be soon (only just started therapy a few months ago). They're now 9 and 5, and both have lots and lots of the items on the RAD lists.
I also yell at them far more than I'd like. Your concern about worrying that you've added to her trauma and made this relationship unsalvagable is one that I often feel as well. We just finalized their adoptions though, because our therapists (one for each child, and one for my husband and me, all at a place that specializes in working with complex blended families/touched by adoption/foster care) all keep telling us that we're making such great progress. Sometimes it feels like we are, other days it doesn't at all.
Someone said that to parent a kid with RAD, the parent has to have completely processed their own feelings of grief and neglect and all that. While I agree that that would be swell, I do think that healing is possible. I've been parenting for 3 years now and am finally coming to a better place . Kids trigger everything, especially at this age and with trauma, they find every single trigger. I'm not sure if it's possible for a person to be aware of all these triggers without having kids! I've really relied on our therapists a LOT over the past 3 years, and join in others in highly recommending that.
For a quickie read on discipline, that's worked well for us, read through the 1-2-3 Magic book. The only thing I would caution you with, with both that method and others that are more "discipline-focused," is that your niece is unlikely to have learned some very fundamental lessons about behavior. Many methods assume that the child already knows how to act, and has the skills to act appropriately, and is therefore willfully misbehaving. While sometimes that's true, I think especially when parenting children who joined us later, we simply don't know what they've learned so far. Much of it is probably wildly inappropriate. So we do add in a lot of teaching, a lot of talking, a lot of reading books together and talking about the character, etc.
Has anyone talked with you and your wife about parenting the child to whatever age they're behaving, rather than their chronological age? My daughter (niece by birth) can go from acting like an 11 or 12 year old, helping care for my friends' toddlers, playing with them, leading play with large groups of kids, reading stories to siblings, etc. But then she can suddenly get triggered (having "big feelings" we call it) and be lying on the couch, sucking her thumb, kicking her legs and wailing like a 3 year old. In those moments, we treat her like a 3-year old. We scoop her up, we hold her, empathize with her, then give her some space to re-center (which has been a huge job for her in therapy). Yes, sometimes it's "just" for attention. Is it so bad that she wants my attention? We try really hard to distinguish between behaviors that are intentional and need to end (whining is a biggie!), and regressions that are a result of her being flooded and unable to cope. I've realized that a lot of my own triggers are around control and grief, and I'm working really hard to deal with my own moments of being flooded, by taking breaks (both in the moment and in a more proactive way).
Lastly, I really hope you and your wife can find a way to come together. If it wasn't for my husband's unconditional support and faith in me (and our joint faith in God, I must add!) I wouldn't have made it through the past 3 years. We're currently in a really tough spot with them, but I'm starting to see the light. I'm finally able to believe that their placement with us is indeed the best for them, even on the days that I yell and get mad and don't handle myself in the ways I know they need. We're finally in a place where most days they know I love them, and I know they love me.
Like others have said, only you and your wife can make the decision of whether you want to commit to parenting her niece, knowing what you know now and what ou're learning. None of us know what we'll get when we start parenting someone, there are no guarantees, but I think that by following our own paths to healing and wholeness, we can make a significant impact on the children we parent. And I say that with all the empathy and love and acknowledgment of how much it sucks to go through that process.