I think Fain can shapeshift and we only saw a few aspects of his abilities during the most bizarre scenes in the early series. Mordeth is a spiritual force that took over Fain's body - I can see Mashadar attempting to escape using Sammael's channeling capabilities in a similar way. It's all rough, of course, but I think this would be very similar to what Fain was doing during TGH when he took control over a force of darkfriend, Fades and Trollocs using intimidation and his own Mashadar powers. Rather than have to kill Fades to intimidate Trollocs, Fain/Mordeth could more easily take on Sammael's physical body - especially since we were expressly told by RJ that the destruction of Shadar Logoth did not weaken Fain/Mordeth.

I recall during TGH that Fain somehow made Machin Shin clog all the gates that Rand and his party tried to use, hence my idea that it may be connected. I think Mordeth/Mashadar is trying to supplant the Dark One using It's/His own forces.