You can till the date your change of status from H1B to H4 is approved. H4 EAD may take more time or may be approved with H4 COS.
No. You can keep changing status as many times as you want. Its good for USCIS as they get revenue -)
Yes, once your H4 COS is approved. You dont need to file for new EAD with H4 COS application if you have one valid till 2023. However note that you can only use it if you are in a valid H4 status.