Jesus speaks of the Brass serpent lifted up prefigures His being lifted up on the Cross.[John 3:14 & 15 ] The brass serpent did not give eternal life it did not even prevent death from an asp bite. BUT the faith in, believing it delivered from death in the desert healed the believers. They didn't eat the Serpent, they just believed the promise of God.Jesus speaks of manna in the desert as prefigure of Him as the Bread of Life. These are prefigurings, metaphorical connections, not literal means to being saved. The serpent of brass did not save anyone bitten by the asps. It was their faith in the propitiatory offering that saved them.
You argument falls flat:
“They didn’t eat the Serpent, they just believed the promise of God.”
They were never commanded to eat the bronze serpent. The followers of Christ were commanded to eat Jesus flesh bu Jesus Himself in John 6 and the Last Supper stories and that was prefigured by the eating of the Passover lamb in Exodus.
“These are prefigurings, metaphorical connections, not literal means to being saved.”
Except Jesus could not have been speaking of eating His flesh as metaphorical since we know the ancient Hebraicism for eating someone’s flesh meant to despise them (see Psalm 27:2).
Best book on the subject is Sungenis’ http://www.amazon.com/Not-Bread-Alone-Historical-Eucharistic/dp/1579181244