a) The DVR is providing the pivot, put simply if the DVR is not connected there would be no access to the LAN.
b) Port Forwarding is opening a port through the firewall from the WAN to LAN. Therefore (in a domestic setup) if that LAN device is compromised there are no other layers of protection from other LAN devices.
c) Yes
d) There are no extensive tests on any DVR/NVR?
Most of these are on a P2P network which I would agree are highly likely to be more vulnerable.
I mentioned my opinions of the flaws with P2P DVR's, a while back.