I am almost as certain that if the FBI really wanted the whole story, the NSA could have arranged a very plausible "discovery" of a backup made by someone just following SOP that was somehow "missed" in the Clinton directed clean-up (aka evidence destroying) sweep. Who knows, it was not only possible, maybe they did give the emails to the FBI and they ignored them. As you say, it is plainly obvious Comey is compromised. No doubt he could have, would have, and quite possibly did direct his agents to destroy and ignore the NSA data dump.
Implicit in your post is the assumption that NSA actually has the emails now.
Given the current tyranny, there is a very strong likely hood NSA was ordered at gunpoint to destroy all traces