has the darkone's energy been limited to only Moridin, or are there more that could use it? ???

For the time being, only Moridin is allowed to use it. He is the only one crazy enough to want to use it anyway.

28 is the number of people been allowed to use the True Power. The thirteen known forsaken and fifteen others during the AoL or perhaps some Dreadlords during the Trolloc Wars. That info is from Moghedien's POV, so it's facts limited to what she knows though.

 

As Majsju states, only Moridin is allowed to use the TP now, it seems to be a privilege of being Nae'blis.

I believe that the seals were still to strong for anyone other than Ishameal to have had access to it during the Trolloc Wars.  Ishameal almost certainly did, hence his advanced saa when we met him, but he was still partially trapped in the seal.