It seems GFE is still having problems. The reason I was trying to install the newest driver before in the first place was the ShadowPlay status indicator was not showing onscreen like it does sometimes, and when that is the case despite ShadowPlay being turned on, I get no capture file writing when I hit the Instant Replay key.
So I resorted to turning Privacy Control off to see if the status indicator would show on the desktop, and it did, and showed in game too. I captured the segment I wanted to and it checked out fine when I watched it, however when I exited game, the status indicator was no longer showing on the desktop.
Then I went into ShadowPlay and turned Privacy Control back on, then off again, and now it's showing. Nvidia needs to sort this out because this has been another ongoing problem for a LONG time.