Thank you so, so, so much; you've been a huge help!!
We're definitely booking Gritti Palace (I have some SPG status as well and don't mind paying for upgrades or using points for specialty suites) so the only thing to consider now is dates; I think we'll likely be able to schedule it for October '15. I'm so excited!
7 nights in Venice, 7 nights in Vienna, 3/4 nights in Salzburg, 1 night aboard the Simplon O-E, and 3 final nights in Brussels!