If Charles wanted to wipe out his entire line then he’d have to persuade his sons to abdicate or become Catholic, and to christen their children Catholic.
good grief can you imagine the look on Kate and her mother's face if that happened.
If it was a choice on regent then definitely Edward , or Zara Tindell ( calm, steady, married succesfully, older) .
Harry - a bit unknown, seems quite emotionally fragile, but and he would have lost his father and brother in this scenario, but he may step up.
But the fact is it is not about who is appropriate or who the public think is appropriate , there are laws that govern this stuff.