Depends on whether you take the Drizzt trilogy as canon.
If that's the case, Obould's kingdom borders right outside Mithral Hall's front door (forgot the name of the Vale) and one side of the river (dang, forgot the name again but i think its the Surbin ?)
But the SM is the latest info that you'll get which doesnt indicate that Obould has actually invaded down south and is still in Dark Arrow Keep.
Another potential assassin of Alustriel could be a Shar cultist that uses the Shadow Weave. The Chosens are quite suceptible to that. You can also use the Dark Moon monks for the stealth bit.
'How did they enter the Palace ?'
Their secret hideout could be the the old mines that was detailed as an adventure in the Silver Marches supplement.
I just thought that the recent Shadowdale adventure would fit perfectly into a Silver Marches war campaign.
It showed that Sharrans could do something about the Chosens and take them out of the picture for awhile. At least that gets to make the PCs the real stars in such a campaign.
Anyone adaption plans for it so far ? It most certainly fits your bill of an attempted assassination on Alustriel.