Speaking as an essentially atheistic Goddess Who is agnostic regarding Her own divinity...
I see no way for anyone to reliably determine if a god is or is not "the real GOD." Are we just supposed to take an alleged god at its word when it says "Yes, I'm the real one; all others are phonies/demons/lesser gods."
We're just not in a position to know whether the purported deity is being forthright with us, because we do not possess the ability to verify its claims. For all we know, it could honestly think
it's the One and Only but be no more than a long-lived extraterrestrial.
That said, the OP is chock-full of unsupported assertions. Whom_seek_ye, we need something a tad more substantial. Empirical evidence to support your claims would be nice.