From the horse's mouth. All the stories are interconnected and discrepancies can be explained away by dream quests and moonbeam roads. Reality is always changing.
The Making of a Sorcerer #1
As I mentioned earlier. Something you gain during dream quests stays with you after you wake.
And even while you sleep it can manifest in reality, and protect you.
As for why Elric doesn't keep Stormbringer after getting it in his earlier dream quests it's because one of the Elemental Lords sealed it away before he woke. It was Grome in specific.
Also taken from the first issue.
Then a thousand years later Grome still remembers their meeting and compact. Elric however doesn't remember because as I mentioned before all that remains is the
power you gain. Your memories of it dissipate over time but the magic stays forever.
This would be issue five.
The Making of a Sorcerer #2
Normal worlds and ghost worlds are differentiated again. They are not the same. Also, travelers on the moonbeam roads don't just walk to these worlds. They also create them. Dream quests constitute all of reality, and there's even a fictional version of our world among them. When the dreaming stops everywhere all of the multiverse disappears. As the multiverse is the product of all dreams. Dream questing is specifically the power to make "the unreal into reality." As I established before.
The best part is that Elric does all of this
without Stormbringer.