What did Yisro mean when he said "Viy'hi 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: He meant that he should be the medium between the people and Hashem, to pass on His teachings to them.
Ramban #1: He meant that he should Daven on behalf of the People.
Rashbam: He meant that, concerning the Dinim that they need to consult Hashem - the "difficult things" mentioned in Pasuk 22, Moshe should take over and hear Hashem's answer.
Targum Onkelos and Yonasan: He meant that he should mediate between Hashem and the people in all matters. 1
Ramban #2: He meant that he should be in the Ohel Mo'ed 2 so he can ask Hashem when necessary.
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 Eilai Lidrosh Elokim" (Pasuk 15). And when, in the following Pasuk, he told Moshe Vehizharta Es'hem Es ha'Chukim ve'Es ha'Mishpatim", he also conceded to what Moshe said there "Ve'hoda'ati Es Chukei Elokim ve'Es Torosav" (Ramban).