Hi,
no need for a real discussion because it's pretty straight forward 
(Check What does AdGuard Home do? )
More secure than not using it.
Risk: If you configure it wrong, it will break VPN policies.
Only DNS can be blocked. So it won't block YouTube Ads or other embedded stuff in websites if they are not served by its own domain. External DNS is always used, that's how DNS works.
Yes, if you choose the same DNS server like your VPN.
No.
Totally.