Here "prophets and the law" refer to the things that the prophets and Moses wrote in scripture. AT: "for these are the things that the prophets and Moses have prophesied through the scriptures until the time of John the Baptist" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The word "he" refers to John the Baptist. This does not mean John the Baptist is literally Elijah. Jesus means John the Baptist fulfills the prophecy about "Elijah, who is to come" or the next Elijah. AT: "when the prophet Malachi said that Elijah would return, he was speaking about John the Baptist"
This is a way of referring to everyone there who was hearing what Jesus was saying. AT: "Whoever can hear me" (See: [[rc://en/ta/man/translate/figs-idiom]])