There is no link between the firewall and RAW sockets. For ethernet it can also put your NIC into a promiscuous mode, so that it will forward to IP the traffic that not only is link-layer addressed to it, but to others on the same segment. Of course, libpcap allows for no way of actually stopping packets from propagating and no way to inject. In fact, libpcap is in many ways orthogonal to divert sockets. There are also accounting chains, but they are of no consequence here.
Forward chain is traversed only by the forwarded packets.
This may sometimes create problems for the interception if you are interested in a certain type of packets that may or may not originate on your host. A lot of times it is not clear which chain to use. As a rule of thumb, forward chain should only be used to filter packets that are forwarded and are not originating and are not addressed to your host. If you are interested in a combination of both forwarded packets and packets that are originating or addressed to your host, then use input or output chain instead. Intercepting on forward and input or output chain for the same type of packet at the same time will create problems in reinjection and, more importantly, is unnecessary.