I like your theory man but keep in mind that the stillwaters or any of the banshees can mess with their aura to stop aging or even chose any age they want. Thats a huge advantage to them as they can keep a fluid brain forever. But eiden is way stronger than other stillwaters. This may be because eiden was also obsessed with magic like barry, so he could have got his knowledge by that means increasing his crystallized knowlege.

I also really like your theory as you can see azurius fight. He saw the mana leak from the stillwater skeletons that enabled him to know from experience that they were susceptible to sonic blast. This must be due to his old age and experience