Each time I start up the controller on my Windows 10 PC and log in I will see the AP is going through provisioning and then connecting. Why is this, shouldn't the provision take place only after the AP is adopted on initial setup? I have a strange thing going on when this happens and I do not know why it is related. My Nvidia Shield (on a wired connection, not using this AP) will lose it's connection during provisioning. Not sure if this is a DHCP talking issue or what but I find it strange. 

Basically the Controller is designed to be online 24/7, this is required if you want roaming between APs.     Therefore when you shut down your PC and the controller is offline, the APs just continue to work, broadcast and operate as previously instructed.

When the PC boots and the controller loads, it will force a new fresh config on ALL the devices and this is what is happening.  During that you will lose connection, its expected.    The controller doesnt know what has been going on while it was offline so just forces the latest settings to everyone, that the cause.

Two more questions...

1) Why do I need see a option in the settings for the 160Hz channel? I only see 20/40/80.

The 620 is a 2x radios 5ghz (each 600mbps max, total 1200), as 600mbps can be achieved on 80mhz width it doesnt support / need 160mhz channel widths.    Faster APs in the 3000+ speeds will require the full 160mhz widths

2) Is there a way to disable the use of any DFS channels at all times? I see a option for Full Sector DFS and this settings leads me to believe there is times this AP may try to use DFS channels. 

The AP wont use DFS unless you select / tell it to do so.    Full Sector DFS just means its supports all the DFS channels and not "some of them".   If you don't set the AP to use DFS it wont use them, instead sticking to channels  36  40  44 and 48 (which are non DFS)