Can anyone provide an argument that provides justification for the existence of evil while taking into account the unnecessary evils, or gratuitous evils, that we seem to observe throughout our life experiences?

Your objection is a good one, but I think we do have reason to believe God might create the sort of world where gratuitous evils are possible. When we consider this problem I think we ought to be wary of placing direct blame for the existence of these evils on God. God may have created a world where these evils are possible, but that does not entail that he is directly responsible for their existence. For example, if I sell you a car and you crash it, it's true that without me you wouldn't have gotten into a car crash, but it is not true that I am to blame. In the same way, I am suggesting that God made gratuitous evils possible, but that does not mean we should blame him.

My argument is as follows:

1. If God exists, then he might create the best possible world.
2. The best possible world would include the potential for gratuitous evil.
a. The best possible world would include the potential for genuinely good actions.
b. If an action is genuinely good, then it was performed by an agent acting freely.
c. So, the best possible world would contain agents acting freely.
d. Agents who act freely have the potential to commit gratuitous evil.
3. If God exists, then he might create a world with the potential for gratuitous evil.

All my conclusion states is that a world with gratuitous evil is not inconsistent with the existence of a God. So,

could that being exist without having created anything? For instance, is the following scenario coherent: God exists and billions of other minds exist that God did not create?

^ This may also be the case, but it could also be the case that both gratuitous evil and God exist. My argument relies heavily on free action being required for genuine good. I think this is an intuitively appealing view though. How could there be courage without the possibility of cowardice, or generosity without the possibility of greed? Certainly, God could've created a world where gratuitous evil was impossible, but it seems like genuine virtue would also be impossible and such a world would be no better, if not worse, than our own.

God cannot create such a stone. But this is not a limitation on his power because "a stone that cannot be lifted by an omnipotent being" is a logical contradiction, a non-thing. Just like an unmarried bachelor or a square circle. Speaking about these things is essentially speaking gibberish because they are non-things. And so not being able to create the stone, or create a square circle is no limitation, because they are not even things. They are non-things.

Hope this was helpful!