"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.
You know, come to think of it, Greeks don't have Beatitudes in the Second Antiphon. So, while the "core" is the same, some things are not. Somebody made that change. For instance, the exist after the Divine Liturgy is also different in Greek churches. In Serbian and Russian churches, we kiss the cross and the priest's right hand. In the Greek churches I have been to, the priest gives out blessed bread, etc. In the Tokyo church, one could buy little biscuits and write names of the people who are sick or departed. That bread was give out at the very door of the church, by name.
So, liturgical practices are not uniform and someone somewhere down the line had to make changes, invent new ones, etc. You are correct that when +Leo III made his pronouncements on the filioque the Creed was not recited in the Latin Church during Mass, but was incorporated into the Mass in 1014 at the request of the Franks.