Get a professional in. There are going to be very few people with experience in databases over a few TB and an 800TB has to be one of the biggest SQL Server databases around.
I'd suggest approaching Microsoft and seeing if you can get the CAT (customer advisory team) people involved. Or contact Paul Randal and see if he'd be available for some consulting
With a database this big, this active and as important as you indicate, you should be looking at getting the absolute top people in to do a professional and comprehensive job. Relying on free help for something like this is recklessness at best.
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