Will it be the Mashiach who will resurrect the dead of the Jewish people when he comes or will it be HaShem?

G-d will resurrect the dead Himself and not through a messenger. As it says in the Talmud (Tractate Taanith 2a) R. Johanan said: Three keys the Holy One blessed be He has retained in His own hands and not entrusted to the hand of any messenger, namely, the Key of Rain, the Key of Childbirth, and the Key of the Revival of the Dead. The Key of the Revival of the Dead, for it is written, “Then you shall know that I am the Lord (and not a messenger – Rashi), when I open your graves” (Ezekiel 37, 13).