I would use fondant. That way you only have to mix the color once and not have to mess with matching the color of the fondant and gumpaste. Since the flowers don't need to "stand out" from the cake there is no reason to have them be harder than plain fondant. If you really feel they need to be stiffer, I would add a little tylose to your fondant.
would also use fondant, same reasons as above.
That is a very pretty cake. I too would use fondant with a little tylose added.
The cake is due the 13th when would you make the fondant and flowers?
You can make them whenever you have time. Once they are dry, store them in a sealed container and they will last a very long time!