If the i485, EAD and AP applications for a dependent is pending with USCIS. Does those petitions get impacted if primary applicant move from an H1b status to EAD?
No, however they lose their H4 status and so if your spouse is working on H4 EAD, then she will need to stop working. Once you start using GC EAD, H4 will transition to pending AOS status and all the GC applications will keep processing without any problem.
For H1b to transition into GC (AOS pending) EAD from the non-immigrant status, Does the dependent have to have an approved EAD (AOS pending)? Or can it be something that is still pending with USCIS?
No, EAP/AP/AOS application may be pending and they still transition to pending AOS status. H4 is a derivative status of H1B and so once the primary stops being on H1B status due to user of GC EAD, the H4 dependents will lose their non-immigrant status.