R6300, wireless and wired, outside subnets can't route to and through this device

Keep in mind that all of your routers have firewalls that restrict devices on the WAN side from accessing devices on the LAN side.  As far as the R6300 is concerned, hosts on "B" are in the Internet and are not permitted to unilaterally contact hosts on "C".  OTOH, hosts on "C" can reach hosts on "B" because firewalls by default permit communication from LAN -> WAN.

Your setup is far from ideal for a typical home network.  You have 3 hierarchical layers of firewalls. Traffic for hosts on "C" is going to be triple-NATed, which is never a good thing for trouble-free communication or speed.  Likewise, traffic for hosts on "B" will be double-NATed.  Unless you have a specific need for such a setup, you should eliminate subnet "C" and, optionally, subnet "B".  You can eliminate subnet "C" by converting the R6300 into an AP.  Likewise, you can also convert the WRT400N into an AP.  As APs, the WAN ports should generally not be used, so you may have to rewire the connections.  If you convert both the WRT400N and R6300 to APs, then you'll be left with just subnet "A" and all hosts will have 10.1.10.X addresses.  You will also get rid of all double and tripled NATing.

Alternatively, if you have a need to segregate hosts on "A" from "B" (e.g. "A" hosts are untrustworthy DVRs ), then you can keep subnet "B".  But traffic from "B" will still be double-NATed.

A third alternative is to enable bridge mode on the DPC3939B (may not be possible without the assistance of your ISP) and convert the R6300 into an AP.  This leaves you with one subnet.  Note that bridge mode will effectively eliminate subnet "A", so "A" hosts will have to move.  But if "A" hosts are, in fact, DVRs then moving them behind either the WRT400N or R6300 may break IPTV.  There are solutions for this but let's not go there unless you are sure you want to do this.