Yes, you can add VLAN ID, Fab Button (yellow) / Advanced / VLAN

1. Deco analyse and work with best channel possible alone. Not good ideia sometimes but in many cases is ok. 2. Always all decos and networks are in same channel, 802.22 k/v/r amendments control all...

You activate "802.11r" Fast Roaming ??? If yes, forgot and register again wifi network into apple device. If no, try activate.

Just open Deco APP/yellow fab button/Wifi: you have password field there, click on EYE at right side and see registered password.

yes, you can do in ap mode, not in router mode.

yes. you can, but P7 need to be configurated first and M9 as sattelite