Thanks for the tip about swipe vs. tap & hold. Swiping usually works in this instance. But what I still don't understand is why, on both my phone and my tablet (both KitKat), the dismiss now options sometimes displays on its own, without swiping, and at other times swiping down is necessary to see the option. What baffles me is that one behavior or the other usually persists for several days or more and is always the same at the same time on both devices at the same time. I still think that somehow it relates to Google software updates being pushed to my devices, but I still have no real evidence that this is the cause.
I think you are searching for the swipe down gesture to uncover, don't tap and hold just swipe down.
The other notification actions I do know about are a press and hold to get a dialog box regarding app info. This is especially useful if you don't know where a notification came from and I know this came to be from apps throwing up notification spam, now you can track which app is responsible for a particular notification and blame the right app if you don't want it.
Swiping left or right does dismiss the notification, and you are correct that unfortunately there is no undo. I don't think I'd want two actions to get rid of a notification, that'd be annoying.
Also I know about the dismiss all button.
Are there any more things that can be done with notifications?