The problem with these examples is that they weren't used in national security systems, which are the systems for which NSA has a legislated defensive responsibility.
Clipper was designed for use by the public; it was not intended to ever be used to protect classified (or even sensitive unclassified) information at all.
Likewise with Dual_EC_DRBG. The CSfC component requirements drew from the Common Criteria Protection Profiles, where Dual_EC_DRBG was never an option.
There is no hard evidence that it was backdoored.