Just out of curiosity – where do you want your close friends to get their long dresses from? If you want them matching, do you want to go to a traditional bridal store and have them (or your parents) buy dresses for them, or are gemach dresses OK, even not matching?
If you don’t expect them to pay extra $ for your enjoyment, then go ahead and help them pick their dresses from a gemach or let them go as a group and pick something nice out that they can agree on and enjoy your pictures after the event.
I would not agree with having them walk down the aisle, as the chassanahs that I have been to in America (my nieces and nephews) only had sisters or nieces going down before the kallah, no friends and none of them were much past Bas Mitzvah, so no big problem with the kehal ogling them.
As far as white dresses – that is a Jewish custom as it is Yom Kippur for the chosson and kallah and they wear white to reflect that.
As far as standing up for the kallah, here in EY, all of the men go out with the chosson and all of the women go out with the kallah. We all stay standing for the whole ceremony. The chairs in America are just to make things “more comfortable”, probably started by the people running the hotels/halls, but if you want to show respect, maybe stand on the side of the chairs.
As many others have said, Mazel Tov and you should be zoche to a bayis ne’eman b’Yisroel.