Not that Adam WAS immortal, but that God was prepared to OFFER him immortality as a reward for obedience - it COULD have been thus and thus was how God ultimately desired and preferred things to be.
God certainly didn’t prefer or desire for man to fall into sin and become inevitably doomed to death, though he knew that it would happen, given man’s weakness and Satan’s powers of persuasion.
It could just as easily have been, and more in line with Christian teachings about the nature of God, that things happened exactly as God ultimately desired them to happen.
How do you reconcile an omnipotent (all powerful, no constraints upon Him other than those that are self-imposed) God who is omniscient (knows every possible outcome of every action before the action is even contemplated, by Him or by humanity) who would set up the circumstances as they are/were, with a God who is also purportedly omnibenevolent (desiring nothing but good for His creations)?
Why would an omnibenevolent being knowingly create a human and place it in a situation (created by said being) wherein it is inevitable (but only, one must presume because the omnipotent, omniscient being desires it to be so for some reason) that the human will contravene laws set up by the omnipotent, omniscient being and then be doomed to death by the same omnipotent, omniscient being?
I simply cannot see how, if one presumes that there is a singular omnipotent, omniscient God, that that God is also omnibenevolent. Two out of the three possibly, but not all three.
An omnibenevollent deity who was omniscient but not omnipotent could desire something for humanity, but be unable to change the situation for some reason.
An omnibenevolent deity who was omnipotent but not omniscient might set into motion situations whose outcome He is not able to predict, though being omnipotent, could, one presumes, change those that did not work out as planned (brings to mind the story of Noah, actually).