영어 어법 질문 몇 가지 드려요

영어 문법 질문이 있어요.
“Sorting your laundry is the first step to helping to keep clothes, linens, and other household items looking their best through repeated washings.”에서 “to helping”의 to는 전치사인가요? “the first step to help to keep”라고 써도 되나요?
“But it was until I was at Cambridge that my father noticed and took me to the family doctor.”에서 that은 강조 구문의 that이라고 하는데, 왜 what이 아니라 that이 답인지 궁금해요.
every처럼 단수 명사나 동사를 쓰는 표현에는 어떤 게 있나요? each도 단수로 쓰나요?