You are a wise mom to recognize that going back and laying a firm foundation is preferable to continuing to build on a shaky one that keeps collapsing. [One of my pet peeves with reading programs that focus on sight reading or whole language while giving phonics short shrift is the future spelling problems that often follow.]
While at first glance going back a few grade levels seems cumbersome--oh, no, I have to do three years of spelling!!--in reality, it doesn't take three years to cover the material. Rather, the best approach is to do exactly as you've done: to go back and test until you see that she is beginning to make more than two or three errors, and begin there.
By administering perhaps three tests a day, you will quickly find out her level without overwhelming her, and can begin at the point where she makes those two or three errors per lesson.
Also, you might wish to test her on the spelling list for each new lesson before beginning the lesson; if she misses only one or two, discuss those words and the phonics rule that goes with them, add them to the next week's list, and move on to that week. Praise her for her advancement!
It appears that she could probably begin with Speller B. However, it is imperative that, when she misses a word, the error is analyzed to find out why she missed it. If the error is related to a lack of understanding of the phonics involved, be sure to add at least a few words that follow that rule to her list.
For example, if she spells stove as stov, it is probable that she never learned long vowel with signal e construction. [Week Seven, Level B] So that rule would be discussed, and a few words from that lesson added to her spelling list.
Remember, too, that CHC's program is flexible; your daughter doesn't have to catch up in her first year of homeschooling! In public school she'd probably never catch up; at home under your astute care, it will happen.
May our good Jesus bless and guide you as your begin this great homeschooling adventure.