Proof that there is only 1 God I'd like to make a critique of this argument. One issue is the association of a God with omnipotence. This argument has the implicit premise that a God must be omnipotent, and indeed, it was stated before the argument was laid out, and in P1 and P2. The argument then associates the idea of omnipotence with that of an inability to be killed, as demonstrated in P5. This opens it up to counterarguments from a multitude of polytheistic religions.
In ancient Greece, for example, multiple Gods were worshipped. Now, in this instance, if each God was stated to be omnipotent, your argument could be suitable: after all, these Gods were noted to have physical bodies, and to engage in physical pursuits (eating, running, and even getting headaches are all noted to be experienced by the Gods in Greek mythology.) Beings with physical bodies, generally speaking, are able to die or have their existence ended in some way. Therefore, if these gods had physical bodies, you could infer that dying would be a possibility for them, which would pose a problem to their omnipotence. However, that claim would be problematic, because the Greek Gods were never considered to be omnipotent, as demonstrated throughout Greek mythology: none of them were all-powerful, since there were Gods for different subjects and areas, and they were able to outwit each other.
For argument's sake, and to illustrate this, let's say that I tell you that I have a religion with ten gods, and none of them are omnipotent. Your argument is laid out in such a way that this would be impossible if the gods were omnipotent (as they would cancel out each other's omnipotence in a difficult paradox), but this example slips through a loophole: none of these ten hypothetical gods are omnipotent. I'll put this example in a standard argument form.
1. If a God does not necessarily have to be omnipotent, it poses an issue to this argument, which would not disprove the existence of multiple Gods.
2. Greek, Norse, Incan, and Mayan religions throughout history, to name a few examples, all involve Gods with limited powers, who do not have to be omnipotent.
3. Therefore, this argument is called into question, and the issue of omnipotence does not necessarily disprove the existence of multiple Gods. (1, 2 MP)
Further, if I change courses and say that I worship a religion with three Gods, and specify that they do not have corporeal forms in any way: no physical bodies, no atomic makeup, nothing, your argument does not contain language that could prove me wrong. P4 gives the impression that death is something that could be applicable within the context of a God's existence, at least enough to create a problem with a God's omnipotence. We don't know, and when formatting an argument, shouldn't assume that a God is capable of dying, particularly in the absence of concrete evidence of a God with a physical form. If a God doesn't exist in a physical body, and isn't "alive" by our definition of the word, it seems implausible to say that "death," again by our definition of the word, would be possible for them. The definition of omnipotence, or being all-powerful, does not necessarily need to concern or specify with regard to death at all.