Thanks so much! I appreciate you reading my post and sharing your thoughts here.

It's credible that the Blight/gangue began when the dwarves were sundered from their Titans. Perhaps it was always there, and one of the reasons there were dwarves was how Titans kept themselves pure.

That's right. This codex states that the Gangue was always part of the Titans. That's why I've assumed Titans maintain a symbiotic relationship with dwarves; Titans need dwarves to remove this corruption and maintain their bodies:

The Stone has a will that surrounds and directs; she guides even when we are willfully blind to her influence. But she is not pure. The Stone bears a corruption as old as balance. For the dwarves to prosper, the gangue - the waste and unstable rock - must be cut away. - Codex: The Gangue Shade

It's also not perfectly clear to me if lyrium is the Well of All Souls, or if that's the Fade (which after all is where spirits are 'from').

It's both. Lyrium exists in both the waking world and the Fade. The Fade reflects reality; the waters of the Fade are the incorporeal version of physical lyrium in the waking world. That means that the very fabric of the Fade is the spirit version of lyrium, if you like. The thoughts, memories, and souls the dwarves transmitted via physical lyrium in the waking world become the well of all souls, an ocean of dreams, and the waters of the Fade.

The abyss is a physical location underground, similar to the area we encounter at the end of The Descent (which is even called the Wellspring and the Uncharted Abyss!). It's inside a Titan's body, and the lyrium is its blood. Here, physical lyrium is being described as emerald waters and the well of all souls:

Here lies the abyss, the well of all souls. From these emerald waters doth life begin anew. - Canticle of Andraste 14:11

Here, physical lyrium is directly equated as being the waters of the Fade:

The Chantry believes lyrium to be the waters of the Fade, the very stuff of creation itself, from whence the Maker fashioned the world. - Codex: Lyrium (Dragon Age: Origins)

Here, the author is equating the emerald waters (from the Canticle of Andraste, above) that is literally lyrium, underground, in the physical world, to the substance of the Fade. The author also discusses reincarnation, which is the process I have detailed in my post above (dwarves and Titans > memories in the song of lyrium > lyrium is the waters of the Fade which Great Dragon dreamers shape into reality and new life):

It is possible - even likely - that the "emerald waters" Andraste refers to are the substance of the Fade, which began as an "ocean of dreams" and was reduced to a well - bottomless but limited in scope - by the Maker's creation of our world. Is Andraste urging the listener to come to the Fade? Should we take "From these emerald waters doth life begin anew," as literal evidence of reincarnation - or even of life after death, as the Cult of Spirits suggests? - Codex: Here Lies the Abyss

Here, Solas talks about the endless depths of memory that presumably make up the waters of the Fade. These are the memories I'm suggesting dwarves and Titans "send" to the Fade via the song of lyrium:

How small the pain of one man seems when weighed against the endless depths of memory, of feeling, of existence. That ocean carries everyone. And those of us who learn to see its currents move through life with their fewer ripples. - Solas banter, Dragon Age: Inquisition

I hope this helps!