I guess it's time for me to respond to my own post
I really appreciate everyone chiming in, and I always enjoy hearing everyone elses' opinions.
Firstly, I'll report on what we decided to do. Rather than removing Lexi, we decided to use a different marker when Lexi made a mistake. We had been using "What happened?" in a somewhat friendly manner. We changed to saying a short, more negative noise, kinda like "unnh". Well, we tried it for one, quick session. It was terrible. I actually saw Lexi start to worry. My little, confident, crazy girl got worried. She hesitated at the weaves. She stopped mid entry and looked around. It was hard to watch. We immediately stopped.
Now, in response to everyones' posts...I don't think that a negative consequence for a behavior is always wrong. In fact, I think sometimes it is necessary, even in its most simplest of forms (I still mark it when Lexi does it wrong). But, you have to know your dog. And the best part is that you can decide what works for you. I tried something different, and it didn't work. So I'm trying something else. Clearly removing your dog is something that has worked for many people on here. And from those who mentioned it, it definitely doesn't seem to have negatively impacted them!
One thing that I do disagree with several other members here about is that every mistake the dog makes (or most of them) is always the handler's fault. Yes, I trained my dog. And yes, I made mistakes. And yes, the dog makes a good portion of their mistakes because of me. But, I also believe that my dogs makes his/her own mistakes. My dog should know to do things without my explicit instruction. My dogs have a responsibility to do the things that they know how to do. And yes, my dog knows how to get into the weaves. She just doesn't care about the details. So in this case, it's her fault.
It's true that somewhere down the line I allowed her to miss some details, or placed more emphasis on other things...but that doesn't mean that I didn't train them. I trained weaves. She learned weaves. I emphasized speed. But that doesn't mean that she's allowed to miss the weaves because she doesn't want to slow down. Partially my fault? Yes. My dogs' responsibility to complete? Yes.
To come back to the weaves, and detail work...right now if Lexi misses an entry, I mark it, and we try the sequence again. I reward Lexi close to my body right before the weaves (lined up so that if she continued she would get her entry). We then try the sequence again. I also try to do a better job of setting her line to help her. I'm not totally clear on our training plan, but I've already seen improvement doing this.