Just a few thoughts.
First, in this context "immortal" really means "not having to die anytime soon because the body ages, wears out, or otherwise fails (dies)." (Nemo's definition).
to be strictly "immortal" in the purest sense of the word, you would have to wait forever to know if it were still alive. Not really a testable sort of thing!
Reminds me of the fellow who fell off the Empire State Building and kept noting as he fell, "So far so good."
So, in this context with "immortal" we are really just referring to getting a body that stops aging and continues living if not hit by a comet, squashed by a steamroller, or otherwise wiped out.
Second, it is no problem for the Satanist to view the physical body as a machine. No need for any supernatural features. Just a machine.
Now you can replace the parts in a classic car as long as you can get parts. In that sense a car, or any machine, could be viewed as "immortal" if you can replace the parts as you go.
It actually requires
a belief in a supernatural means to prevent
the improvement of the physical body such that it could not
be improved to not wear out or permanently fail (die).
To deny that such improvements are possible
, you would have to believe that a supernatural God doesn't want that to be possible.
Otherwise, if the body is just a machine then sooner or later science will be able to keep that machine running as long as you wish it to.
Third, for Satanists the appeal of an unaging, undying, and youthful body is a no-brainer.
The people who say today they would reject physical immortality are like the fox who said the grapes he could not reach must be sour. For them, immortality is unattainable and unreachable. Just science fiction.
For Cambridge gerontologist, Aubrey de Grey, it is nothing more than a biological research issue involving the repairing of the seven physical damages that aging causes.
Your last questions answer themselves when rewritten slightly:
Now should the day come when one would live forever how would you go about it and would you even want to?
Now should the day come when one could DIE tomorrow, how would you go about it and when would you even want to?
At what price would you be willing to cast aside an immortal life ?
At what price would you be willing to cast aside DEATH?
Wait around. You might just get to answer those questions for yourself. "Life is the great indulgence, death the great abstinence."
-The Satanic Bible