Yes, mc1 and mc2 are both on the root timeline. The similar issue is that using a relative path does not work. Its been awhile since I used Flash and could do wonders with AS 2, but AS 3 is hard to get used to. (second nature to start typing _root...). I was told to avoid absolute paths and vars, and use events. What is the best way to have a movie clip play and then tell another (sibling) movie clip to play when it reaches a certain frame? When mc1 stops on frame x, the code below runs. The commented line is the one that does not work.
var fl_SecondsToCountDown:Number = 1;
var fl_CountDownTimerInstance:Timer = new Timer(1000, fl_SecondsToCountDown);
is not a correct relative path if it is coming from inside mc1. "this" refers to the current timeline, and that would be the timeline inside mc1.
Using "this" is very often unnecessary, especially in AS3. A correct relative path to mc2 would be "mc2" if you are in the tieline where mc2 lives, as in... mc2.play(); there is no need to use "this" at all.
As far as the orginal complaint of this posting goes, which refers to the using the Actions panel to define paths... the safest solution is to not use the automatic script tools provided in the Actions panel and learn how to code without Script Assist. I remember somewhere beyond a decade ago when I first started using Flash I tried using the Script Assist feature and was quickly frustrated with it... so we parted ways and I never looked back.