If the group doesn't want to leave the city before the battle is over, Xath can always take L'Aurel herself to Caer Maelyn. The ressurection is going to take some serious negotiating, something at which Xath excels, but which I know the others in the party are not too keen on. Limiting the number of party members traveling to Caer Maelyn eliminates the need to sneak off to the portal stone. However, leveling also eliminates the need to sneak off to the portal stone.
Characterwise, I'm a little iffy about the party leaving the city during the battle, but I also know that L'Aurel needs us, and were I she, I'd want to be in that waiting room for as little time as necessary. L'Aurel has always been there for Xath, and now is one time Xath can be there for her. So Xath will be going to Caer Maelyn in four hours. And she'd go sooner if she could.