Welcome to the forums, Sarah. To answer your question, Jesus is God, creator of the universe, and nothing is impossible for him. The miracles recorded in scripture attest to that.
At Mass we re-enact the Last Supper and the death of Christ. The offering of the body and blood at Mass comes before the point in the Mass recalling Jesus’ death (when the priest breaks the Host just before Communion).
We do things in this order because that is how Jesus did them. As the previous person said, there is no requirement that Jesus die first before He consecrated the first Holy Eucharist.
The most common understanding is that the bread and wine at the last supper became his body, blood, soul, and divinity as it was at that time. They were still partakers of the divinity and of the lamb of God.
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