If it's a matter of a backdoor/special key the FBI must keep secret, it will leak, period. Then what?

If it's a matter of having the computing resources to crack a key, then people's computers are too easily compromised to act as bots to solve any such problem. Our computing infrastructure is simply too vulnerable. To avoid this scenario, you'd have to secure every operating system in the world in a way that they could not be compromised, even in principle. That's as close to a proof as should be needed, unless there's a compelling counterargument to suspect some assumption is false. I have yet to hear such an argument.

It will leak, period? What's the Dual EC private key?

1. "I'm not aware of it having happened, therefore it has not and will not happen". I won't even bother listing all the fallacies in that position.

2. Dual EC isn't protecting anything of value, and hasn't in about 15 years. If your reply is meant to be pedantic about my phrasing, notice that the context is about encryption that is actually in use.

It's not protecting anything of value, you mean, besides every VPN session generated by the most important vendor of hardware VPNs in the industry prior to 2016?