I think this issue would happen with any routing protocol right? Not just EIGRP.
I think the discard route being referred to is the standard route to NULL0 that gets automatically created by the router originating the summary as a loop prevention mechanism.
The root of the problem, as I see it, is that A has a single point of failure when reaching the C, D, and E subnets. So, of course, if A is advertising a summary and A looses one of those links then it's going to blackhole the traffic for sure.
The only way that I can think of to fix this is to install a redundant additional link between A & B so A has an alternate path to C, D & E and eliminate the single point of failure. Otherwise, you would have to come up with some convoluted tracking object that if any of the links between A and C, D, & E go down then stop advertising the summary.
I'm curious if anyone has any other more "elegant" solutions.