For the killswitch, just have all of your firewall rules that route your traffic to the VPN use the VPN interface as the gateway, if the gateway is down then the internet is down.
For the DNS leak I think this setup would be best:
Do what you already did (select VPN interfaces as only outbound interfaces for DNS resolver)
Go to General Setup and check "DNS Server Override" but leave all of the fields blank (also leave all other DNS fields blank, don't put your VPN providers DNS, google DNS, etc. in any of these fields, all of them totally blank).
On whatever clients that you don't want using VPN DNS, create a static IP, and enter the DNS server that you want that client to use.
In this setup everything will resolve to Root servers via the VPN by default. All clients that you create a static mapping for will resolve to whatever DNS server you assign via WAN (assuming you didn't force them through the VPN with firewall rules).