Oops yea ur totally right about rouse. I just read extend and for some reason thought hey worked similarly. Even if you we're to use extend it would leave the grapple creature stunned and incapacitated.
However It does say a grappler can grapple mutiple creatures for each attack that has he grapple trait. But don't know for sure if any the creatures in PvS has a a second grapple attack it sounds like.
Here is the text from he codex below
A creature can be the grappler in several Grapple conditions (it can hold onto multiple preys), as long as it has a separate attack with the Grapple trait for each prey it is holding. In such a case, place all of the prey cards next to the grappler, with a separate Grapple condition on each one.