Bill, darling, breathe.
I swear I didn't hear one single break for breath in those two paragraphs.
First thing: what sorts of event scenarios do you anticipate in the coming two years? No point working up a huge back story if it's not going to be useful for your events. Most history-heavy events will have room for a range of impressions, but I would encourage you (and Sister Bill) to work on something middle-of-the-road so she can jazz it up for higher things, and tone it down for lower. That's generally more flexible than one static role, and will allow maximum event participation.
5'11" is a tall girl. In order to keep from looking like Alice After The Mushrooms, her cage is going to need to balance with her figure; it may need to be a bit larger than 90" for a good finished look. Nice starched petticoats will help fill things out for balance, too.
A cotton wash dress is useful for many things; keep that in the wardrobe. She can use it as a "dressed down" style for middle class or upper working class impressions, and as a normal everyday dress for active working class impressions, as the event calls for them. The sleeve style should be just fine with her age (it's fashionable, and 15 tends to pull toward fashion), though you could absolutely cut it into a short puffed sleeve if she's interested in something cool and comfortable for summer use. Not every dress has to be a year-round dress, after all.
Do you have enough of the windowpane to do a full dress out of? A nice white sheer would be ideal for a summer dress, or even a fall/winter/spring party dress, so she could get a lot of use out of one, and it lends itself nicely to colored silk waists, sashes, etc. Anna Allen has a lovely waist (Swiss waist is another term you'll see) that she's worn with several fine dresses, and it's a great accessory for changing the look of a sheer white windowpane dress.
If you don't have enough windowpane for a dress, may I encourage you to do the skirts in silk taffeta, and reserve the tropical wool for a dress all its own? Silk has a look to it that just hollers "Gosh, I did up this ensemble RIGHT."
Keep in mind there are bits of a middle and upper class wardrobe you'll have to come up with, too: fashion bonnets and a parasol among them. And shoes/boots... something fashionable, not utilitarian.
I've probably just added more questions into the mix.
Feel free to continue this thread, or start a new one as you see fit!