How can the dependant become adjustee before primary? Simple: by using the EAD that was provided by THEIR I-485. Remember, each family member submits their own I-485.
That, as you have said, doesn't make then an adjustee. Using EAD or AP does.
So, for example, I stayed on H1 for many years after my wife used her EAD, relinquishing her H4 status. So the derivative has NO effect on the primary's non-immigrant status. However had I used my EAD or AP, she would by force become adjustee, no longer eligible for h4.
The same applies (in a shorter time frame due to TN non-dual situation): A TD spouse using her EAD does not affect the primary's TN. The primary's use of EAD does kill any TD',s.