What did Yisro mean when he said "vi'Yhi Elokim Imach"?
Rashi: He meant that Moshe should consult Hashem before accepting his advice.
Ramban (citing Ibn Ezra): It was a request that Hashem should help him to succeed in carrying out his advice. 1
Rashbam: He was assuring him that if he accepted his advice Hashem would be with him and he would be able to survive.
And it is only later in Pasuk 23 that he instructs him to consult Hashem (Ramban citing Ibn Ezra).
What did Yisro mean when he said that Moshe should be for the people "Mul ha'Elokim"?
Rashi and Seforno: That he would be the medium between the people and Hashem, to pass on His teachings to them.
Ramban #1: To Daven on behalf of the People.
Rashbam: Laws that they must ask Hashem about them, i.e. the "difficult things" mentioned in Pasuk 22, Moshe will hear Hashem's answer.
Targum Onkelos and Yonasan: To mediate between Hashem and the people in all matters. 1
Ramban #2: He will be in the Ohel Mo'ed so he can ask Hashem when needed. This is not where monetary cases were judged.
What are 'the matters" that he advised Moshe to bring before Hashem?
Rashi: The people's quarrels (that he was unable to solve).
Conceding to Moshe's opening response to his initial query "Ki Yavo Eilei Lidrosh Elokim" (Pasuk 15). And when, in the following Pasuk, he told Moshe Ve'hizharta Es'hem Es ha'Chukim v'Es ha'Mishpatim", he also conceded to what Moshe said there "Ve'hoda'ati Es Chukei Elokim ve'Es Torosav" (Ramban).