We know that the Realms as a place to tell stories is older than D&D, I just can't recall if the "Down to Earth Divinity" article I mentioned upthread talks about what sort of Cosmology, if any, Greenwood created for the Realms prior to his work adapting his world to the D&D Cosmology as presented way back in the day.1) The multiverse "Great Wheel/Planescape" universal D&D cosmology predates the Forgotten Realms.
Enough with the trying to win the conversation, hey?There is. Canonically.
The Realms is its own setting--always has been--and while it's Cosmology has been presented a few different ways over the last 30+years, off the top of my head I don't recall any Realms sourcebook that says, "If you're not sure what to do with leftover souls, check Planescape."
Can you?
I do know of several Realms sourcebooks that have the subject of the souls of the dead covered, however.
It's worth remembering the Realms original design had it as a nexus between many worlds and realities. That's as Ed Greenwood intended it. Easier to jump a party of adventurers between worlds for a change of pace every now and again, and to introduce things from other realities (including from Earth, and just about any other place you can think of beyond what's in the Great Wheel), all for use in the Realms.
It was also Greenwood's intention that the extraplanar realities and worlds beyond the Realms remain uncertain at best, unknowable at worst. The accounts of adventurers, the writings of planar travelers (of which the Realms has had plenty), the musings of sages...these were the best that could be had. (Note: this is how Greenwood "turned the page" on the Great Tree in one of his recent Realms novels; he's a big fan of the Great Wheel).
Why? So DMs had leeway to do their thing, without having to worry so much about players shoving a rulebook in their face and saying, "If you want to deal with FR as it is actually presented, you have to include the entire rest of the multiverse, because FR is a subset of the rest of that multiverse, according to FR itself."
It's not so cut and dried as this.There's the rest of the multiverse.
There exists a few decades of gaming material that presents the Cosmology of the Realms in different ways, so there are different generations of gamers who view and think of the Realms differently.
So gamers should ignore the multiverse, if it so pleases them, because the return of the Great Wheel does not mean all the old pre-3E information has returned to a place of primacy, just as some of us in this discussion prefer to ignore it.
Planescape material is an option and not a requirement for participation in this discussion.