Spike's variation of his original Cecily Poem could also be a statement of Spike and his life coming full circle again - he loved and lost his first life motivated by love, pain and sorrow but he also finds a new life through  his "raven colored clothes" which is Drusilla/Death/Black Kali.  Spike meets both death and life in that alley with Dru.  With Buffy he reaches out again for Love and again lives in pain and with sorrows -  but Buffy directs him back toward the Light and the Sun; the universal Life Force that is inseparable from its counter part, human death.   

A very important part of this poem I think is the connection of his love for Buffy and the Initiative chip.  It's that chip as well as his capacity to love that  together eventually create the en-souled Spike.  The reason that I give the chip much importance when connecting with Drusilla and Spike's transformation is her "little fishes" (paraphrase) lines - The Fish in the Sea are another symbol of man's development and changed life. 

Your observation on the use of "soft" are excellent -

From Legen - Scarlet Ibis:

"Now clearly, this could be a reference to the amulet, literally cutting a swath of glimmering gleam.  But also it is a representation of Spike's love, which brought him there in the first place.  His love--his soul--is the light in this underground."

"As all of this glorious light is emitting from Spike, he says he can feel his soul--he knows without a doubt it's really there.  His eyes grow wider, and he kind of smiles, even though it stings to feel it in such a way.  It is after this that Spike decides to "see how it ends," inspired from the beauty of William's soul."

This description of his feeling of his soul and the little stinging, from my perspective tells of the very strong connections that vital parts of William and Spike shared.  While his physical body was going to burn to ash and into the amulet, his soul/spirit seemed to easily exist within his Spike/Vamp life form - he did not suffer huge intolerable pain, but his transformation was gentle and perfectly attuned with The Sun Life Universal Force Cycle of birth and death.  That's why Spike can find joy while his death is right there with him, he is perfectly in harmony with human life.

Spike-William poem can be interpreted as prophetic, but also as a description of his life journey  from Cecily-Drusilla-Buffy-Africa and at the face of Life-Death Cycle. 





 

i can't recall how much of the poem is said in ffl, but barring that the whole poem isn't still about cecely (My soul is wrapped in harsh repose; Midnight descends in raven-colored clothes she was kind of a bitch, and while he did "love" her, i'm sure he was a smart enough guy to see that), just completed 200 and some change years later.
My soul is wrapped in harsh repose the time that he was a vampire, cause his soul wasn't there and yet it was a little.
Midnight descends in raven-colored clothes drusilla
But soft...behold life as a vampire wasn't that bad, in fact it made him a better man...pire
A sunlight beam Cutting a swath of glimmering gleam. buffy
 My heart expands, 'Tis grown a bulge in it Inspired by your beauty-- Effulgent how much he loves buffy, and changes for her.
i wouldn't put it past william, or the writers, to write a prophetic poem. but on a different note, as i stated at the begining of this post, we never knew what the real beginning of the poem was. maybe it's a completely different peom that spike wrote, and he just thought the lines were still good and reused them.
you never know in the buffyverse.
p.s. thank you so much for posting the poem si. :)