Agrarian, Pope Leo III consented to filioque being sung but not inserted into the Creed because the Creed cannot be altered (in writing). However, every source I looked at, including Romanides', leaves no doubt that +Leo stated to the effect that filioque was theolgically sound and that he agreed with it. As to why he agreed with its theology is a different story.
Romanides has his own theory, which very plausibly takes the political situation and the reality of Frankish barbaric mentality into account. Others simply read nothing into it, nor try to paint the picture in the context of times, but just state tat Leo agreed theologically with filioque, while proscribing its insertion into the official Creed.
Romanides makes a valid point when he says that +Leo III specifically explained to the Frankish legate that the Fathers and the Councils did not "miss" inserting the filioque as a diplomatic way of saying he in his heart did not believe it to be theologically sound but could not afford to draw ire of his Frankish protectors.
I tend to side with Romanides, because contextual understanding of historical events is indispensable. However, the true reaosn remains unknown and only subject to educating speculation.
"Pope Leo III consented to filioque being sung but not inserted into the Creed because the Creed cannot be altered (in writing)."
Very odd. Have you seen this anywhere but in that quotation from Romanides -- i.e. have you read independent accounts of the "singing vs writing" that make clearer what is somewhat obscure in Romanides?
Your reading of Romanides, then, is that Pope Leo thought it was fine to sing the Creed (ad lib, I guess) with the filioque in the Liturgy, as long as it was never put into writing? This would seem to make little sense -- by this logic, a priest or bishop could make whatever changes to the services he wanted, as long as they were not theologically incorrect, and as long as what was actually written in the books wasn't changed.
"However, every source I looked at, including Romanides', leaves no doubt that +Leo stated to the effect that filioque was theolgically sound and that he agreed with it. As to why he agreed with its theology is a different story."
Yes, in what looking I have done, everything either remains silent on his view of the theologically soundness of the filioque, or it states that he privately thought it was OK. I have found nothing that specifically indicates that he disagreed with it theologically. But as you say, and as Romanides articulates, the context and circumstances (the man went through some *very* rough and dangerous times) do reasonably point at his holding a view of the filioque that Orthodoxy, then or now, wouldn't have a problem with.