I can't speak for others but I've always felt that each spell has to be assessed on an individual basis (which is what you seem to have done). Casting a magnitude three spell that creates an earthquake or activates a volcano could be said to run for an extended amount of time, whereas a spell such as demonic blade makes sense that it just runs for the combat.

With regards to having two blades - considering the spell description does mention that the "spell often requires Obvious Technique" I would suggest that the sorcerer has to concentrate and actually use gestures to move the sword; the advantage is that the sorcerer isn't under threat of immediate harm.

Again, these are just my own opinions and others may have far better suggestions.

It seems that you have a pretty good handle on things. At the end of the day it's a matter of doing whatever works best for your group and for the campaign; I figure that was likely the original intention behind the way the magic rules were written.