KT:
The "navy" dress is the new "little black dress". Consider it, as Tracey has said, as a neutral. (choc Brown is also the "new" black).
While lavender and white can work, lavender is a receding color, so in her photos, the lavender will appear to be a little dull -- almost grey -- and washed out. Pairing it will a brighter flower (hot pink, lime green, yellow, orange, etc). will help to make the lavender not so dull.
Consider the paring of: lime green, hot pink and orange -- it looks terrific against navy. Bring your navy into the bouqet thru the accents of dark blue delphinium.
Your orchid dilemna and the round bouqet -- you could, very carefully, wind three stems of orchids around the outside of the bouquet. Bridesmaids, however, are not careful people, and I would fear that the rough handling of the bouquet by the BM will make the orchids snap in two.
I would, as Queen and Tracey have suggested, remove the orchid florets from the stem, cross-wire each blossom & tape, insert as many as you need to into the bouqet. This will allow you to have a "rounded" shape.
Don't forget that the dendrobium also come in a solid fuschia as well as a fushia/white verigated. Very bold and would look good with your lavender roses.
Hope this helps!
Cheryl