May a woman be appointed wasi?

Agreed. She may. Ibn Qudama states that appointment of a woman as wasi is valid “in the view of most of the people of knowledge” and names Malik, al-Shafi’i and the Hanafis among them, before stating the Hanbali ground for it himself: ‘Umar ibn al-Khattab appointed his daughter Hafsa as his own executor, and a woman is competent to give evidence, so she is competent to hold the office. He deals directly with the obvious objection — that a woman does not sit as a judge — and answers that judgeship is a different office with different requirements, so nothing follows from it for the wasi. The only dissent he records is ‘Ata’, a Successor, and none of the four schools follows him. We checked this point; the schools do not differ on it. The conditions that do apply — capacity, integrity, Islam, ability to do the job — apply to a woman exactly as they apply to a man.

Shared evidence: al-Mughni, Kitab al-Wasaya, fasl 4773 — “the appointment of a woman is valid in the view of most of the people of knowledge… and Malik, al-Shafi’i and the Hanafis said so”, with the report that ‘Umar appointed Hafsa · al-Fiqh ‘ala al-Madhahib al-Arba‘a, vol. 3, “the chosen executor” — none of the four school lists of conditions requires the wasi to be male.

Not covered by this finding: the separate offices a woman’s appointment does not carry with it — sitting as a judge, and acting as marriage guardian — which the sources treat as different questions with different rules — not yet researched school by school. Treat that as open and ask a scholar of your school.