i played with it a little on the poor mindspore thralls in corathus creep.
as far as i can tell, say you've got a group of 5 mobs, you sing Creeping Dreams on one, and he gets mezzed for 12 seconds. the next tick (6 seconds later), the surrounding mobs get mezzed, if they don't resist the check. now all 5 are mezzed. 6 seconds later, the initial mob wakes up from his mez and resumes attacking. 6 seconds after that, another ae-mez seems to go off. so it appears that the song mezzes one target, then mezzes multiple targets in a radius from the initial target, two times.
will i use it ? that's debatable. in a zone where i don't need to keep Selo's memmed, freeing up a song slot, maybe i'll keep it loaded for those times when multiple mobs attack.. i can sing this, then begin single target mezzing and if they don't resist the two mez tics, the situation might turn out okay.
but my normal lineup doesn't have the room for two mezzes. and with the recast timer on it, its too dangerous to keep this loaded over, say, Aelfric's, or Luvwen's, or Morrel.
as for the spell missions, these ones are actually difficult. 68.2 is pretty tough. 68.3 is just awful, it takes forever ot kill all the mobs you need, and then you get to sneak up and fight the chamber guardian. 68.4 is a long mission, although its fun. and 68.5 is easy but the mobs are beefy.. lots of HPs.
i say, do the 69 arc, get thousand blades, and leave the others.