A background in various occult and magical practices would be useful here, obviously. Unfortunately I can't provide that.
One thing to consider though, the story of the sermons is the story of a world falling apart - might the cornerstones be related to establishing (or reestablishing) order?
I was only thinking about the tower stones that are believed to hold things together - I had not thought about replacing them or reinforcing them before ... nice one
What were those red, thorny plants in the Ashlands where the Redoran Ald'Rhun is?
So is there a progression like: animal, vegetable, magica, mineral? Assuming wind/aether/magica correspond.
I had assumed for some reason that the cornerstones were the Daedric Princes or their Realms ... add one = ?
But there again if this is aimed at Dunmer then they would more likely correspond to locations in Morrowind - (specifically Vvardenfel) maybe to Daedric Shrines - also can the cornerstones also be aligned with any of the Houses? Redoran would follow Boetithiah I suppose ... etc
Looking at magica & stuff:
Cornerstone one has a finger
Buried under, pointing through
Dirt, slow low in the ground
North cannot be guessed,
And yet it is spirit-free
plants/ingredients do not have spirits ... Alchemy
Cornerstone two has a tongue,
And even dust can be talkative,
Listen and you will see the love
The ancient libraries need
the love of writing/reading - love of knowing = philosophy ... Scrolls
Cornerstone three has a bit of string,
Shaped like your favorite color,
A girl remembers who left it there
But she is afraid to dig it out,
And see what it is attached to
object/s - cat's cradle? - spirit-linked ... Enchantments
Cornerstone four has nine bones,
Removed carefully from a black cat,
Arranged in the fashion of this word,
Protecting us from our enemies
bones do have spirit links; - what "word" is this? ... Necromancy / Ancestor spirits
Not sure about this at all ... maybe you see some sense in this matrix or you can add to or alter it meaningfully?
does someone have an idea about a Dunmer or Daedric rune/word this could be - in which case they might all have an associated rune
A last thought as this is Vivec - what about the structure of Vivec City? Could that be relevant?