Ok, the ultimate question is, where does it say "go back to the normal rules" when the child turns 18?
I finally found it in the regulations, even though it is not in the tax instructions or publications. It's example 6 in section (g) here,
https://www.law.cornell.edu/cfr/text/26/1.152-4
Example 6.
F and G are the divorced parents of Child. In May of 2009, Child turns age 18 and is emancipated under the law of the state where Child resides. Therefore, in 2009 and later years, F and G do not have the right under state law to physical custody of Child for over one-half of the calendar year, and Child is not in the custody of F and G for over one-half of the calendar year. Section 152(e) and this section do not apply, and whether Child is the qualifying child or qualifying relative of F or G is determined under section 152(c) or (d).
That means go back to square 1, ignore everything else.
Since your daughters live at home with you, they are your qualifying relative dependents. They must answer "yes" to the question "Can someone else claim me as a dependent." If you don't want to claim them as dependents because you are income-limited on the AOTC, they can answer "no" to the second question "Will the person who could claim you actually claim you this year?" They are disqualified from the recovery rebate but may get something on the AOTC.
Sorry for all the confusion, but at least I know where it actually says "go back to the standard rules" since it doesn't say that anywhere else I can find.
