Thanks for all the advice, Saintheat.
Since my players are all pretty decent optimizers, I think I will start them at level 4; doing a total xp count, that sets their levels as something like:

After Vraath Keep - 4
After Skull Gorge Bridge - 5
After travel to the Blackfens - 5
After the ruins of Rhest - 6
After the Ghostlord's lair (and travel to it) - 7
After the battle of Brindol - 8
After the Fane of Tiamat - 9

So basically, they'll be 1-2 levels behind throughout the entire campaign. But honestly, with double the 'standard' number of players (and optimization to boot), I think they'll do okay.

And you make a good point about the redspawn arcanissi. Especially since Karkilan will be stronger than normal, I think I'll put just one in (and, if the party is having lots of trouble, possibly not include it at all).

Again, thanks.