Julie, thanks for this reminder. I don't know why my mind was blocking and thinking that the student would be finished with ILL
before 6th grade. But you're right, the last 1/3 of the book is for 6th grade!
Schlee, there are two different ways to approach the study of grammar -- whole to parts, and parts to whole.
Using Charlotte Mason methods (which MFW does) is a whole-to-parts method. That is, the students gets sort of a "global" study of grammar through literature and
using the language, beginning with reading well-written works (reading to learn), copywork, dictation, and narrations, both oral and written. One is learning
how the language is used in its context studies of history, Bible, science, and other subjects... and actually using it themselves. These are very, very effective methods for both oral and written communication when one is older, as it's "ingrained" into their brains, so to speak. To use an analogy, it works sort of like a timed-release capsule: It's working its way through your system and taking effect over a period of time, not instantly. You don't necessarily see
immediate results by having the child take a test at the end of so many lessons and producing a grade, which may or may not then transfer over to proper usage in real life.
The parts-to-whole method, however, is the focused, intentional study of grammar which gives everything a name. It teaches
why we must have both a subject and a verb that reflect the same tense in order to make a complete, comprehensible sentence. (Although we may inherently know this because after years of practicing the whole-to-parts every day, we know something's wrong with a poorly written sentence, even if we don't know why.) The analogy in this case is taking something that gives us immediate relief, i.e., Tylenol instead of that timed-release capsule.
We need BOTH. Charlotte Mason (and thus, MFW) believed it was mostly fruitless to try to force the parts-to-whole method down a child at an early age because their brains weren't developed enough yet to comprehend the why, and that it was just as, if not more effective to wait until the child's brain reached a level of maturity that it made sense. And this would be after several years of utilizing the whole-to-parts so that the why makes sense. This is exactly what happened with both of my older girls. Every time I attempted "formal" grammar (parts-to-whole) with them when they were younger, they just didn't get it... didn't care or see the point of it (because it was out of context), and so it mostly ended up being busywork which made ME feel good because I had them "doing grammar". However, somehow through the years, they managed to not only become good readers, spellers, and communicators, but they came to LOVE the English language, and even developed a
desire to study it more formally. That one year I mentioned above when I taught both of them from a R&S English book.... they were in 10th & 7th grades that year. My oldest was also doing a year of Latin that year, and she told me later that that was her best year ever for learning English. Prior to that, even though she
did it, she didn't
understand it. (The introduction to Latin helped her better understand *English* grammar, as well.) All of the "formal grammar" books I'd done with her before that were just a frustration to her. Could or should I have done R&S with her much sooner? Probably. R&S is an excellent program that teaches very, very well. But I guess I didn't think I needed to since she
was doing some formal grammar here and there, and was a voracious reader, a good speller, and loved to write. Like your dd, mine loved/loves etymology, too. She refers to diagramming sentences as "organizing words".
After 10th grade when she did R&S English and an intro to Latin course, she flew through Spanish I very easily, with all A's. (She and her sister both took Spanish with another group of homeschoolers, and her teacher is someone who's taught at the college level for many years.)
So I did all those things a little
too late with her (and she only got in one year of Spanish before graduating), but my point is that they do NOT need years and years and years worth of formal grammar repeated again and again during the elementary years in order to be excellent students and communicators. She's living proof of that. She got nearly perfect scores in Reading and English on the ACT, was told by the history teacher at community college (who taught solely by lecture, note-taking, and essays, with very little memorization of names and dates) that she didn't need to take Comp I, as she had received the highest score in the history class and was an outstanding student. She did take Comp I (wanted to for the experience), but she learned
nothing as far as grammar or writing skills in that class. She did learn something about the topics she had to research for the papers, but not the writing itself. She got all A's in that class, too. In fact, the teacher asked for permission to use two (out of four) of her papers as examples in future classes. He told her that he believed her papers would raise the bar for other students' work.
All of that is not to brag on my daughter, but to boast in Charlotte Mason methods for language arts. They really and truly do WORK. Is formal grammar needed? Yes. But not until late elementary or middle school. Miss Mason knew her stuff.
I can give a similar testimony for the methods working on my second dd who's always been what I call a "boy learner", as her preference would be to fill out some workbooks, call it done, and get busy playing or doing something outdoors. She never loved reading or writing or any form of "school" like her older sister did, and her fine motor skills were poor for a very long time. My older dd
loved both PLL and ILL (and then later, R&S). My second dd absolutely
hated PLL and ILL. Why? Because they made her WORK. She had to slow down and focus enough to be able to do the lessons and really think about what she was doing... and she hated that! But I made her do it, anyway. Same with the copywork, etc. that are scheduled within MFW. But she's really good at memorization, so again, she would've preferred to just do textbooks all the way. If I had let her do that, though, I don't think she would love language study as much as she does NOW as a rising 10th grader. She's recently decided that she might like to get a degree in either teaching or English, and
asked for a R&S English book to study on her own. She also had an easy time with Spanish I this year, even though she hasn't taken the Latin course that my oldest did. But she has done EFTRU, she used the same spelling curriculum that my oldest used, she's done MFW for many years (which always incorporates those whole-to-parts methods of copywork, dictation, narrations, notebooking, research skills, and a great deal of quality written work), she did PLL and parts of ILL, and she did
some formal grammar here and there.
Will everyone have the same outcome with every student? Of course not. God made us all differently, with different chosen paths in life. But even STEM students should be able to understand the written and spoken word well enough to properly read, comprehend, interpret, and apply Scripture.
Some people never "love" language study like my girls do, no matter what. And it's not "wrong" to do formal grammar study every year of elementary... but it's not
necessary, either. It's just a personal preference thing. I expect that your daughter, Schlee, will do well with
whatever you choose for grammar next year, partly because she already loves the study of words, and partly because she's a 6th grader, so her brain is developmentally ready for it.
Using MFW since 2004. 
The All in One book that MFW recommends would be a very basic, foundational grammar book that might be good for her to go through just to get familiar with all those terms, and then you could choose something else for 7th grade.