I would cut it down to 5-7 dress/skirt outfits plus a shorts/capris outfit (hiking?). You can wash frequently, if needed. Or even just quick hand wash. You could throw more outfits into her suitcase if there's room or if you detest laundry, but I don't think you really need them.
I think dresses/sandals are fine, as long as the sandals are good-fitting and she can walk a lot in them. My DD lived in dresses/sandals for 3 years and had no problems.
Everything else on your list sounds fine.
I'd definitely bring a warmer option for the plane though - that is a long time to be cold. Maybe the leggings like you mentioned. And socks. Put them in your carry-on until she complains about the cold.
We spent 6 weeks in Spain last summer and definitely overpacked.