Let's start with the fondant...I don't want metallic silver, I want more of a shimmery/pearly light gray. I'd prefer not to paint it as it is going to be a very large cake and I don't own an airbrush. Can I just knead one of those powders (pearl dust or something) into my fondant?

Now the buttercream...If the bride opts for purple, versus the silver she originally wanted (gray cake, ewww), then she'll want silver accents. The idea we discussed over email (meeting with her next week) had scroll work. So again, do I have to paint it or can I mix some sort of shimmer into my buttercream. Also, here I wouldn't want it to be overly metallic (not like aluminum foil)

I would mix pearl dust with some black food color to make silver. That will give you the look you are going for.

You can try the spray cans of pearl luster if you can get your hands on it. It is always on back order or out of stock. That would be your best bet not having an airbrush.

Luster is painted or dusted on top in a small amount. If mixed in it would dissapear into the fondant.

The platimun silver dusts sound like the slivery shimer you are looking for. There are four differnt shades of platinum; Brilliant silver, Silver Satin, Silver Silk and Silver Glitter.

http://lusterdust.com/pldufdaapbyc.html

this is the spray stuff

http://www.fondantsource.com/pepmelusp.html

you might find it at another store