Just to be precise, the authors may be just as trustworthy but even good people make mistakes. The code is definitely less trustworthy because it uses the OpenVPN code plus more GUI code. In addition, more people look at and review the OpenVPN code than the front-end of one vpn provider.
You are in a bit of a pickle because you have limited support options. You're one of a handful of people who use Tor with KVM with a specific OpenVPN client. If it's not a general VPN issue then KVM, Whonix, Tor, and OpenVPN support can't really help you - especially without knowing what client you're even referring to. I would try asking for help from the authors - if it's open-source, you can probably post to the Github Issues page.
Before asking, you can make sure it's not a KVM or Whonix issue by eliminating some possibilities:
With VPN running in host:
1. Does host internet work? check.
2. Does TBB in host work? check.
3. Does internet in a plain Debian-8 VM work?
4. Does TBB in a plain Debian-8 VM work?