This is a fairly interesting problem. And I think that we do not yet have quite enough information to be sure what the problem is but I have an idea. I note in the show ip route that router C leans routes only from addresses .2, .3, and .4 and learns no routes from .11 (router E) or from .12 (router D). I also note that router D learns routes from .2, .3, .4, and .11 but not from router C. And router E similarly learns routes from .2, .3, .4, and .12 but not from router C.
So router C does not learn from either D or E and D and E do not learn from C. This leads me to suspect that there is some layer 2 issue in which they do not have connectivity with each other. I note that your drawing does show links between C and D and E, but I suspect that these links are not working.
It might be helpful if you would post the output of show cdp neighbor detail from routers C, D, and E. I suspect that it will show that they are not seeing each other at layer 2. I also suspect that the output of show ip eigrp neighbor will show that C, D, and E are not forming neighbor relationships which would be consistent with a lack of layer 2 connectivity.
I believe that the output of show cdp neighbor detail is consistent with my theory that there is some layer 1 or layer 2 issue. I am quite surprised that the show ip eigrp neighbor shows as many neighbors on routers D and E.
I note that routers D and E have EIGRP on Ethernet and FastEthernet interfaces and C is on VLAN 254. Perhaps you could post configuration details of the interfaces/ports of each of these?
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...