You have to pick:

If the coin lands on tails and I become omnipotent, I would use that power to reach back through time and make the coin have a 100% chance of coming up tails. So I can't lose. I can find no logical flaws in this plan. :)

That can be the engraving on your tombstone

Well I can. If the flip gives you the omnipotence, then you need not go back and fiddle with the past result. If you get death, you die and that will not change.

Well, you'd be alive in a parallel universe, from that universe (the paralel) you could change the outcome.

but if it lands on heads you will never become omnipotent, unless in some alternate universe

That's how parallel universes work. At least one 'me' would become omnipotent and capable of changing the outcome in the other universes. Flawless.

@andemon

Right, as long as the parallel universe theory is true. Its just a theoretical way to interpret quantum behavior. There are other explanations. There is no evidence for it, except that it hasn't been disproven. The main reason it's popular is because its cool and causes interesting thought experiments, not because its true per-se. I'm not sayings its NOT true, I'm just saying, we have no idea from a scientific perspective.

So yeah, you can have "I believed the multiple universe interpretation of quantum mechanics engraved on your tombstone"

However, if you, or an alternate reality version of you, becomes omnipotent, you could rewrite the laws of the universe to make the parallel universe theory true.

Still only works is alternate universe is true now or you flip tails (in which case you can in fact make it true). If alternate universe is not currently true, and you flip heads, you are still gunna die. There is no escape.