Christy,
The ladies are doing an absolutely tremendous job of talking through your options with you.

I agree fully with their wise counsel as you consider what is best for your daughter.
First of all, I would encourage you to take a good look at the placement chart and decide where your daughter fits all by herself. If you can for now, set aside all thoughts of combining with her younger siblings, and just really focus on where she fits best alone that will give you a better placement for her. It is possible that she may fit in either CTC or RTR, when you consider her all alone.
Next, I'd also encourage you to consider your boys all alone and make sure each of them is best placed in CTC, when taking your daughter out of the equation.

The boys need to be able to read their own history and science material and follow fairly lengthy written directions independently, so you'll want them well-placed for that.
Once you have the correct fit for each family member, then I'd move through the areas where your daughter is struggling and work to get a better fit for her if possible. For example, if she is struggling with the math, and it is taking her a long time each day, throwing her day out of balance I'd lean toward consideration of a different math option. At HOD, we are looking to recommend Discovering Mathematics for those families who desire a more Singapore-based math approach, and we are also looking toward recommending VideoText Algebra for those who desire a more traditional math route.

If your daughter has not come up through Singapore math, I'd be inclined to recommend a look at VideoText for math. There are also many other math options you could consider instead, but I would definitely consider a switch in that area.

I agree that Pre-Algebra in 9th grade is alright for those who are not exploring a math or science related field. Your child can still go on to college when doing Pre-Algebra in 9th. The mention I made of doing Algebra I by grade 9 was in reference to needing that level of math to be able to complete a rigorous level of science in the early years of high school.
Next, I'd take a look at her grammar background, and since you shared that it is on the weaker side, I'd lean toward using Rod and Staff English to help with that. Make sure to do much of each lesson with her orally or on a white board, assigning only one part to do on paper. I'd be inclined to lean toward using Rod and Staff English 5 along with either CTC or RTR for her, as English 5 is considered the foundational year of Rod and Staff. I would work to do all of English 5 in one year with her, and you will see tremendous growth.
Next, I would look at your writing options. I would likely lean toward using whatever writing is scheduled within the program she ends up using. CTC schedules Write with the Best (you could possibly use Vol. II in place of Volume I if your daughter has already had quite a bit of practice in descriptive writing) and RTR schedules Medieval History-Based Writing Lessons.
As far as literature study goes, if she hasn't had much in the way of formal literature study, I would lean toward using DITHR 6/7/8 for her literature portion. Here is a link to possible high school level lit. you could use for each of the genres:
viewtopic.php?f=6&t=7765&p=56997#p56997
As far as science goes, I agree with 8arrows that a switch to Exploration Education Advanced Version for science, as scheduled within Rev2Rev would be a very good choice. If you wanted to do it without the inventor's study within Rev2Rev, then you could just purchase the EE Advanced Kit on its own and use that for her physical science with lab for high school credit, as it is credit worthy on its own.

My own son did something like this for his 9th grade year and enjoyed it immensely.
If you did end up using CTC for grade 9, then her high school cycle would look like this:
CTC - Grade 9
RTR - Grade 10
Rev2Rev - Grade 11 (We would recommend adding government here. There are already suggestions under Rev2Rev for how to do this.)
Modern Times - Grade 12 (We would recommend adding economics/personal finance here. There are already suggestions on the board for how to do this.)
Your world history would be in grades 9-10 with American in grades 11-12. A quarter credit of geography would be awarded over each year, totaling one full credit by grade 12. This is just one way to do it, but it would work well.
CTC for high school:
viewtopic.php?f=6&t=5400
RTR for high school:
viewtopic.php?f=6&t=8143
Anyway, feel free to post back and share your thoughts, after you've taken one more good hard look at the first page mainly of the placement chart. I agree that your daughter will make more progress than you can imagine, once we begin slowly steadily working in the incremental skills needed to make her a lifelong learner. You will find those skills wound right within your guide, with each guide's skills building well on the skills from the previous guide. I would go ahead and start counting credit this year, as this will give you the option of graduating her in 4 years if desired. She will change so much as she journeys, and it is good to be prepared for whatever God has in mind for her.
Blessings,
Carrie