I’m not sure if she’d be open to a girls’ school, but I just wanted to put a quick plug in for taking a look at The Madeira School, in McLean, VA.
I think it hits all of your must-haves, and has a unique offering in their Wednesday co-ops, which might be close enough to the Mercersburg experience to warrant mentioning. Essentially, every Wednesday at Madeira is dedicated to outside-the-class learning, service, and development. Freshman year is high ropes / wilderness education. Sophomore year is focused on service. Junior year they’re Congressional Interns on Capitol Hill in DC. Senior year they intern in a professional setting that they’re interested in exploring more deeply. (When my younger sister was there, I believe her senior year Wednesdays were spent directly shadowing doctors at either Walter Reed or NIH.)
The campus is absolutely gorgeous, and they just received an anonymous $60 million gift — I believe the largest gift ever given to a girls’ school — earlier this year. The girls there have a reputation for being curious, friendly, accepting, and terrifically kind.