Some updates on this issue...
Now you can set step-end and step-start easing to skip a keyframe in the middle, so the animation appears to hide the middle keyframe.
For example, if there are three keyframes in the timeline, and if you would like to make the middle keyframe to be skipped, set step-end for the 1-2 keyframes, and set step-start for 2-3 keyframes.
These are easing settings, so please find them from Right-Clicking the animation span.
When it is previewed, the animation won't have smooth animation, but it jumps from 1 to 3rd keyframe instead.
If the timeline event is used with setting visibility:hidden, please make sure to set visibility:visible for another timeline event marker. Otherwise, visibility:hidden is set for the rest of the animation time.
Note: Visibility change can be only seen on browser preview, not the stage preview.
When double-clicking the timeline event marker, it only shows the blank event dialog, not the event you set previously. We are working on this issue, but in the meantime, please go find the event (e.g. page1>event-1>...) in the Event Panel and double-click the event there to see the existing event.
Please let me know if you have any question,
Thanks!
Mariko - Google Web Designer Team