I replied you via email.

  1. Yes you can. Change in more settings->custom DNS. It works with VPN.
  2. You may or may not. Pls note, if vpn is running on pc, all data, including dns request may just go via the vpn tunnel of the pc and the router cannot detect. Also router cannot override if the pc use encrypted dns.
  3. There is already. Check Cloudflare.
  4. No. The router has to work in Router mode, not bridge mode (AP, extender, WDS).