So, as I was digging through the sources of info and while nothing more concrete about timing, here are a few minor additions.
From Before the Storm:
“He is indeed Forsaken. He seems genuine, though when I suggested he pledge fealty to you and the Horde, he demurred. He said he preferred to serve the Light rather than kings or queens.”
“Ha!” Sylvanas said without humor. “I liberated him to be a Forsaken so that he could have free will, and thus am I repaid. No matter. I take it you believe he is harmless.”
Now, Alonsus himself was not in the story (as undead) till the Legion. So the only organisation I was able to tie him to is Conclave. Which puts him into the category of those who were in the Netherlight Temple. This was a prison built by draenei.
While this says nothing about the timing with Calia Menethil events, it limits the time when Alonsus could even joing the thing - there was no draenei presence on Azeroth till TBC.
Can’t add anything else so far.
edit:
Was this the moment she had been thinking of ever since she had found her way to the Netherlight Temple and Archbishop Faol?
seems to be too weak of a link.
gl hf