It seems that Cinderon and Gnarladon's spawn rate (for invasions) is nil, or extremely low.
Having leveled two toons to 50, and now waiting quite a few hours for either of them to spawn (5ish invasions, all Morkhar or whatever, while at 50), I have only seen them once each (while leveling).
I have been lead to believe that they only spawn when enough people are in the zone, if this is the case, it should be changed immediately.
Now this wouldn't be an issue if they weren't needed for the epic quest line needed to acquire the legendary from Lord Greenscale.
It is also rather frustrating to have to wait upto 2 hours between invasions, and IF (IF!) the boss spawns, they don't guarantee the drop (from the two times I have managed to kill them) due to a bug, tagging or some other issue.