The distinction is between actions that occur before the frame, and actions that occur after the frame.
See in player_actions.js, runFrameActionBefore and runFrameActionAfter each handle a distinct subset of actions.
On a merged frame, the runFrameActionAfter will be skipped (see player.js line 296
In other words, any action which is supposed to run after a frame will be skipped on a merged frame; they can only occur on the last frame of the merged sequence.
This is why the editor also has logic to hide these actions from the list : even if you could select them, they wouldn't be executed in the player.
(Which is exactly what will happen if you set one such action and then tick the "merged" checkbox : the action will still be stored in the trial data, but ignored by the player.)
And yes, the actions that run after the frame are those which prompt for player input and/or trigger a redirection to other frames, because those couldn't work properly in a merged frame.
As for Set Game Over Redirection, yep, I just messed up I guess. It's part of runFrameBefore, so it can work on merged frames, and thus shouldn't be hidden in the editor.