고득점 330제 문장 구조 질문드려요

문장 몇 개의 문법 구조가 헷갈려 질문드립니다.
1. “So little was known about the brain that it was not until the 14th century that its basic function was understood.” 여기서 주어가 무엇이고, so little that 구조가 맞나요?
2. “Some veterans, however, remained refusing the offer.”에서 refusing과 remain의 관계가 궁금합니다. remain의 보어로 to부정사가 쓰이는 경우도 알고 싶어요.
3. “A and B were criticized for this tactic, which they claimed was ***.”에서 they claimed는 삽입된 표현인가요?
설명 부탁드립니다.