Anthony, I don't think you're a 'bad owner'. Nor do I think Kira is a 'bad' dog. Howver, without diligent management & oversight these incidents couold escalate to a truly bad outcome someday. PLEASE don't let that happen. You will have failed Kira big time if you do.
Years ago I had a Sibe that broke house training after another dog joined the household. Cochise was a tricky cuss & I finally in desperation began tethering him to me any time he wasn't confined. I thought he'd hate it b/c he was extremely active & liked almost constant movement. Surprisingly, he thrived on the additional 'attention' & I think decided that it elevated his status that I needed/wanted him with me at all times! Please, consider tethering her. You might find it much less bother than you expect it to be.
At this point, actively discourage protective tendencies/behavior. These qualities are not an asset in an easily stressed & anxious dog. I doubt she'll ever be a good candidate for even informal family protection. Despite that she can be a loving & enjoyable companion if you will help her to be the best that she can be.
Her behavior was (IMO) completely inappropriate & demonstrated a disturbing lack of judgment & discernment. The average dog understands emotional nuance & vibes much better than humans usually. Dogs who are well suited to protecting their people don't react to adolescent/teen annoyance, snarkiness or mild discomfort. They can distinguish b/w arguments, even loud, angry arguments & real threats. They're unfazed by childish squabbles.
The woman was a welcome guest in your home who'd been there at least 2 hrs before Kira went off. In all that you've described, I simply don't see a good reason for Kira to believe your daughter was threatened or in danger. Protective dogs, especially large, powerful dogs, simply MUST be clear headed & capable of impeccable judgment.
Becoming a bit watchful would have been appropriate. Going off on a welcome guest was not. Yeah, she stopped herself, but the fact is that she stopped behavior that should never have even begun in this particular situation. Frankly, that's not good enough.
Please don't take what I've posted as 'flaming you'. You can & should work diligently to protect this lovely girl that you obviously care a great deal about. Unfortunately, there seems to be a serious disconnect b/w your words & your actions. Kira NEEDS you to act in her behalf & see that she is protected from herself.